Coloring the digits of Pi

Here we use tokcycle to smoothly vary the colors from one end to the other, and then pass the resulting token stream to a version of the OP's \pipar routine.

\documentclass{article}
\usepackage{shapepar,microtype,xcolor}
\def\pipar#1{\shapepar{\pishape}\bfseries#1\par}
\def\pishape{%
{25.0839}%
{0.0838926}b{14.3456}\\%
{0.0838926}t{14.3456}{33.3054}\\%
{0.503356}t{11.5772}{37.6678}\\%
{1.25839}t{9.98322}{39.6812}\\%
{2.09732}t{8.52614}{41.5578}\\%
{2.85235}t{7.21477}{42.8691}\\%
{3.27181}t{6.7953}{43.2886}\\%
{4.11074}t{5.95638}{43.7081}\\%
{5.28524}t{4.78188}{43.7081}\\%
{5.62081}t{4.44631}{15.1007}st{19.547}{12.6678}st{32.2148}{15.0168}\\%
{5.62081}t{4.44631}{7.9698}t{19.547}{2.34899}t{32.2148}{2.34899}\\%
{6.04027}t{4.18011}{6.22257}t{19.4227}{2.37488}t{32.1424}{2.37047}\\%
{6.87919}t{3.64772}{5.16101}t{19.1741}{2.42667}t{31.9978}{2.41343}\\%
{7.63423}t{3.16856}{4.04621}t{18.9504}{2.47328}t{31.8676}{2.45208}\\%
{8.05369}t{2.90236}{3.80463}t{18.8261}{2.49917}t{31.7953}{2.47356}\\%
{8.38926}t{2.6894}{3.61137}t{18.7267}{2.51989}t{31.7245}{2.50373}\\%
{9.22819}t{2.15701}{3.12823}t{18.4781}{2.57167}t{31.5474}{2.57045}\\%
{9.98322}t{1.67785}{2.96021}t{18.2544}{2.61828}t{31.388}{2.6305}\\%
{11.5772}t{0.415968}{2.85584}t{17.7821}{2.71667}t{31.0515}{2.75727}\\%
{11.9966}t{0.0838926}{2.91826}t{17.6578}{2.74256}t{30.9629}{2.79063}\\%
{12.4161}t{0.0838926}{2.64861}t{17.5336}{2.76846}t{30.8743}{2.82399}\\%
{12.7517}t{0.0838926}{2.43289}t{17.4088}{2.81003}t{30.8035}{2.85068}\\%
{13.1711}t{0.0838926}{2.22315}t{17.2529}{2.862}t{30.715}{2.88404}\\%
{13.5906}t{0.0838926}{2.01342}t{17.097}{2.91397}t{30.6264}{2.9174}\\%
{14.0101}t{0.0838926}{0.838926}t{16.9411}{2.96594}t{30.5378}{2.95076}\\%
{14.0101}e{0.922819}t{16.9411}{2.96594}t{30.5378}{2.95076}\\%
{14.7651}t{16.6605}{3.05948}t{30.3785}{3.01081}\\%
{15.604}t{16.3487}{3.16342}t{30.2013}{3.18792}\\%
{16.7785}t{15.9121}{3.30893}t{30.0039}{3.3854}\\%
{21.896}t{14.0101}{3.94295}t{29.1434}{4.24586}\\%
{25.0839}t{12.7724}{4.3305}t{28.6074}{4.78188}\\%
{25.8389}t{12.4793}{4.42229}t{28.6074}{4.78188}\\%
{29.0268}t{11.2416}{4.80984}t{28.6074}{5.39494}\\%
{29.4463}t{11.0415}{4.8981}t{28.6074}{5.47561}\\%
{30.2013}t{10.6813}{5.04094}t{28.6074}{5.62081}\\%
{30.6208}t{10.4812}{5.12029}t{28.6074}{5.72383}\\%
{34.9832}t{8.40004}{5.9456}t{28.9901}{6.41263}\\%
{35.4027}t{8.19993}{6.00914}t{29.0268}{6.58557}\\%
{37.3322}t{7.27942}{6.30143}t{29.5346}{7.04256}\\%
{38.1711}t{6.87919}{6.42852}t{29.7554}{6.64702}\\%
{38.5906}t{6.87919}{6.29195}t{29.8658}{6.44925}\\%
{39.3456}t{7.09492}{5.59084}t{30.1612}{5.9965}\\%
{39.7651}t{7.21477}{5.20134}t{30.3254}{5.4129}\\%
{40.5201}t{7.63423}{4.24257}t{30.6208}{4.36242}\\%
{40.9396}t{8.01175}{3.56544}t{31.2081}{2.60067}\\%
{41.3591}t{8.38926}{2.43289}t{31.7953}{0.838926}\\%
{41.3591}e{10.8221}e{32.6342}%
}
\usepackage{tokcycle}
\newcounter{stepcount}
\newcounter{colorindex}
\newcommand\restorecolor{\setcounter{colorindex}{100}}\newcommand\reducecolor[1]{%
  \stepcounter{stepcount}%
  \ifnum\thestepcount=#1\relax
    \setcounter{stepcount}{0}%
    \addtocounter{colorindex}{-1}%
  \fi
  \color{black!60!blue!\thecolorindex!red}%
\ifnum\thecolorindex<1\relax\setcounter{colorindex}{1}\fi}
\begin{document}
\tokcycle
{\addcytoks{\bgroup\reducecolor{5}#1\egroup}%
\ifx.#1\addcytoks{\restorecolor}\fi}
{\processtoks{#1}}
{\addcytoks{#1}}
{\addcytoks{#1}}%
{3 . 1 4 1 5 9 2 6 5 3 5 8 9 7 9 3 2 3 8 4 6 2 6 4 3 3 8 3 2 7 9 5 0 2 8 8 4 1 9 7 1 6 9 3 9 9 3 7 5 1 0 5 8 2 0 9 7 4 9 4 4 5 9 2 3 0 7 8 1 6 4 0 6 2 8 6  2 0 8 9 9 8 6 2 8 0 3 4 8 2 5 3 4 2 1 1 7 0 6 7 9 8 2 1 4 8 0 8 6 5 1 3 2 8 2 3 0 6 6 4 7 0 9 3 8 4 4 6 0 9 5 5 0 5 8 2 2 3 1 7 2 5 3 5 9 4 0 8 1 2 8 4 8 1  1 1 7 4 5 0 2 8 4 1 0 2 7 0 1 9 3 8 5 2 1 1 0 5 5 5 9 6 4 4 6 2 2 9 4 8 9 5 4 9 3 0 3 8 1 9 6 4 4 2 8 8 1 0 9 7 5 6 6 5 9 3 3 4 4 6 1 2 8 4 7 5 6 4 8 2 3 3  7 8 6 7 8 3 1 6 5 2 7 1 2 0 1 9 0 9 1 4 5 6 4 8 5 6 6 9 2 3 4 6 0 3 4 8 6 1 0 4 5 4 3 2 6 6 4 8 2 1 3 3 9 3 6 0 7 2 6 0 2 4 9 1 4 1 2 7 3 7 2 4 5 8 7 0 0 6  6 0 6 3 1 5 5 8 8 1 7 4 8 8 1 5 2 0 9 2 0 9 6 2 8 2 9 2 5 4 0 9 1 7 1 5 3 6 4 3 6 7 8 9 2 5 9 0 3 6 0 0 1 1 3 3 0 5 3 0 5 4 8 8 2 0 4 6 6 5 2 1 3 8 4 1 4 6  9 5 1 9 4 1 5 1 1 6 0 9 4 3 3 0 5 7 2 7 0 3 6 5 7 5 9 5 9 1 9 5 3 0 9 2 1 8 6 1 1 7 3 8 1 9 3 2 6 1 1 7 9 3 1 0 5 1 1 8 5 4 8 0 7 4 4 6 2 3 7 9 9 6 2 7 4 9  5 6 7 3 5 1 8 8 5 7 5 2 7 2 4 8 9 1 2 2 7 9 3 8 1 8 3 0 1 1 9 4 9 1}
\restorecolor
\expandafter\pipar\expandafter{\the\cytoks}


\end{document}

Here's a low tech way to do this. This uses predetermined colours or a randomly assigned color for each digit. The random colour code is taken from Generate random color in way that works with both pdflatex and lualatex.

\documentclass{article}
\usepackage{shapepar}
\usepackage{microtype}
\usepackage{xcolor}
\usepackage{pgf}
\usepackage{pgffor}
\usepackage{xcolor}

\usepackage{ifluatex}
\usepackage{ifxetex}
\newif\ifrandomcolor
% begin code from https://tex.stackexchange.com/a/447066/
\ifluatex
  \let\pdfrandomseed\randomseed
\fi

\ifxetex
  \pgfmathsetseed{\time}
\else
  \pgfmathsetseed{\number\pdfrandomseed}
\fi

\newcommand{\randomcolor}{%
    \pgfmathsetmacro{\R}{rnd}%
    \pgfmathsetmacro{\G}{rnd}%
    \pgfmathsetmacro{\B}{rnd}%
    \definecolor{randomcol}{rgb}{\R,\G,\B}%
}
% end code from https://tex.stackexchange.com/a/447066/

\newcommand{\assigncolors}{
\ifrandomcolor
\def\1{\randomcolor\textcolor{randomcol}{1}}
\def\2{\randomcolor\textcolor{randomcol}{2}}
\def\3{\randomcolor\textcolor{randomcol}{3}}
\def\4{\randomcolor\textcolor{randomcol}{4}}
\def\5{\randomcolor\textcolor{randomcol}{5}}
\def\6{\randomcolor\textcolor{randomcol}{6}}
\def\7{\randomcolor\textcolor{randomcol}{7}}
\def\8{\randomcolor\textcolor{randomcol}{8}}
\def\9{\randomcolor\textcolor{randomcol}{9}}
\def\0{\randomcolor\textcolor{randomcol}{0}}
\else
\def\1{\textcolor{red!10}{1}}
\def\2{\textcolor{red!20}{2}}
\def\3{\textcolor{red!30}{3}}
\def\4{\textcolor{red!40}{4}}
\def\5{\textcolor{red!50}{5}}
\def\6{\textcolor{red!60}{6}}
\def\7{\textcolor{red!70}{7}}
\def\8{\textcolor{red!80}{8}}
\def\9{\textcolor{red!90}{9}}
\def\0{\textcolor{red!100}{0}}
\fi
}


\def\pipar#1{\shapepar{\pishape}#1\par}
\def\pishape{%
{25.0839}%
{0.0838926}b{14.3456}\\%
{0.0838926}t{14.3456}{33.3054}\\%
{0.503356}t{11.5772}{37.6678}\\%
{1.25839}t{9.98322}{39.6812}\\%
{2.09732}t{8.52614}{41.5578}\\%
{2.85235}t{7.21477}{42.8691}\\%
{3.27181}t{6.7953}{43.2886}\\%
{4.11074}t{5.95638}{43.7081}\\%
{5.28524}t{4.78188}{43.7081}\\%
{5.62081}t{4.44631}{15.1007}st{19.547}{12.6678}st{32.2148}{15.0168}\\%
{5.62081}t{4.44631}{7.9698}t{19.547}{2.34899}t{32.2148}{2.34899}\\%
{6.04027}t{4.18011}{6.22257}t{19.4227}{2.37488}t{32.1424}{2.37047}\\%
{6.87919}t{3.64772}{5.16101}t{19.1741}{2.42667}t{31.9978}{2.41343}\\%
{7.63423}t{3.16856}{4.04621}t{18.9504}{2.47328}t{31.8676}{2.45208}\\%
{8.05369}t{2.90236}{3.80463}t{18.8261}{2.49917}t{31.7953}{2.47356}\\%
{8.38926}t{2.6894}{3.61137}t{18.7267}{2.51989}t{31.7245}{2.50373}\\%
{9.22819}t{2.15701}{3.12823}t{18.4781}{2.57167}t{31.5474}{2.57045}\\%
{9.98322}t{1.67785}{2.96021}t{18.2544}{2.61828}t{31.388}{2.6305}\\%
{11.5772}t{0.415968}{2.85584}t{17.7821}{2.71667}t{31.0515}{2.75727}\\%
{11.9966}t{0.0838926}{2.91826}t{17.6578}{2.74256}t{30.9629}{2.79063}\\%
{12.4161}t{0.0838926}{2.64861}t{17.5336}{2.76846}t{30.8743}{2.82399}\\%
{12.7517}t{0.0838926}{2.43289}t{17.4088}{2.81003}t{30.8035}{2.85068}\\%
{13.1711}t{0.0838926}{2.22315}t{17.2529}{2.862}t{30.715}{2.88404}\\%
{13.5906}t{0.0838926}{2.01342}t{17.097}{2.91397}t{30.6264}{2.9174}\\%
{14.0101}t{0.0838926}{0.838926}t{16.9411}{2.96594}t{30.5378}{2.95076}\\%
{14.0101}e{0.922819}t{16.9411}{2.96594}t{30.5378}{2.95076}\\%
{14.7651}t{16.6605}{3.05948}t{30.3785}{3.01081}\\%
{15.604}t{16.3487}{3.16342}t{30.2013}{3.18792}\\%
{16.7785}t{15.9121}{3.30893}t{30.0039}{3.3854}\\%
{21.896}t{14.0101}{3.94295}t{29.1434}{4.24586}\\%
{25.0839}t{12.7724}{4.3305}t{28.6074}{4.78188}\\%
{25.8389}t{12.4793}{4.42229}t{28.6074}{4.78188}\\%
{29.0268}t{11.2416}{4.80984}t{28.6074}{5.39494}\\%
{29.4463}t{11.0415}{4.8981}t{28.6074}{5.47561}\\%
{30.2013}t{10.6813}{5.04094}t{28.6074}{5.62081}\\%
{30.6208}t{10.4812}{5.12029}t{28.6074}{5.72383}\\%
{34.9832}t{8.40004}{5.9456}t{28.9901}{6.41263}\\%
{35.4027}t{8.19993}{6.00914}t{29.0268}{6.58557}\\%
{37.3322}t{7.27942}{6.30143}t{29.5346}{7.04256}\\%
{38.1711}t{6.87919}{6.42852}t{29.7554}{6.64702}\\%
{38.5906}t{6.87919}{6.29195}t{29.8658}{6.44925}\\%
{39.3456}t{7.09492}{5.59084}t{30.1612}{5.9965}\\%
{39.7651}t{7.21477}{5.20134}t{30.3254}{5.4129}\\%
{40.5201}t{7.63423}{4.24257}t{30.6208}{4.36242}\\%
{40.9396}t{8.01175}{3.56544}t{31.2081}{2.60067}\\%
{41.3591}t{8.38926}{2.43289}t{31.7953}{0.838926}\\%
{41.3591}e{10.8221}e{32.6342}%
}



\begin{document}
\randomcolortrue
\assigncolors

{\pipar{\3 . \1 \4 \1 \5 \9 \2 \6 \5 \3 \5 \8 \9 \7 \9 \3 \2 \3 \8 \4 \6 \2 \6 \4 \3 \3 \8 \3 \2 \7 \9 \5 \0 \2 \8 \8 \4 \1 \9 \7 \1 \6 \9 \3 \9 \9 \3 \7 \5 \1 \0 \5 \8 \2 \0 \9 \7 \4 \9 \4 \4 \5 \9 \2 \3 \0 \7 \8 \1 \6 \4 \0 \6 \2 \8 \6  \2 \0 \8 \9 \9 \8 \6 \2 \8 \0 \3 \4 \8 \2 \5 \3 \4 \2 \1 \1 \7 \0 \6 \7 \9 \8 \2 \1 \4 \8 \0 \8 \6 \5 \1 \3 \2 \8 \2 \3 \0 \6 \6 \4 \7 \0 \9 \3 \8 \4 \4 \6 \0 \9 \5 \5 \0 \5 \8 \2 \2 \3 \1 \7 \2 \5 \3 \5 \9 \4 \0 \8 \1 \2 \8 \4 \8 \1  \1 \1 \7 \4 \5 \0 \2 \8 \4 \1 \0 \2 \7 \0 \1 \9 \3 \8 \5 \2 \1 \1 \0 \5 \5 \5 \9 \6 \4 \4 \6 \2 \2 \9 \4 \8 \9 \5 \4 \9 \3 \0 \3 \8 \1 \9 \6 \4 \4 \2 \8 \8 \1 \0 \9 \7 \5 \6 \6 \5 \9 \3 \3 \4 \4 \6 \1 \2 \8 \4 \7 \5 \6 \4 \8 \2 \3 \3  \7 \8 \6 \7 \8 \3 \1 \6 \5 \2 \7 \1 \2 \0 \1 \9 \0 \9 \1 \4 \5 \6 \4 \8 \5 \6 \6 \9 \2 \3 \4 \6 \0 \3 \4 \8 \6 \1 \0 \4 \5 \4 \3 \2 \6 \6 \4 \8 \2 \1 \3 \3 \9 \3 \6 \0 \7 \2 \6 \0 \2 \4 \9 \1 \4 \1 \2 \7 \3 \7 \2 \4 \5 \8 \7 \0 \0 \6  \6 \0 \6 \3 \1 \5 \5 \8 \8 \1 \7 \4 \8 \8 \1 \5 \2 \0 \9 \2 \0 \9 \6 \2 \8 \2 \9 \2 \5 \4 \0 \9 \1 \7 \1 \5 \3 \6 \4 \3 \6 \7 \8 \9 \2 \5 \9 \0 \3 \6 \0 \0 \1 \1 \3 \3 \0 \5 \3 \0 \5 \4 \8 \8 \2 \0 \4 \6 \6 \5 \2 \1 \3 \8 \4 \1 \4 \6  \9 \5 \1 \9 \4 \1 \5 \1 \1 \6 \0 \9 \4 \3 \3 \0 \5 \7 \2 \7 \0 \3 \6 \5 \7 \5 \9 \5 \9 \1 \9 \5 \3 \0 \9 \2 \1 \8 \6 \1 \1 \7 \3 \8 \1 \9 \3 \2 \6 \1 \1 \7 \9 \3 \1 \0 \5 \1 \1 \8 \5 \4 \8 \0 \7 \4 \4 \6 \2 \3 \7 \9 \9 \6 \2 \7 \4 \9  \5 \6 \7 \3 \5 \1 \8 \8 \5 \7 \5 \2 \7 \2 \4 \8 \9 \1 \2 \2 \7 \9 \3 \8 \1 \8 \3 \0 \1 \1 \9 \4 \9 \1}
}

\randomcolorfalse
\assigncolors
{\pipar{\3 . \1 \4 \1 \5 \9 \2 \6 \5 \3 \5 \8 \9 \7 \9 \3 \2 \3 \8 \4 \6 \2 \6 \4 \3 \3 \8 \3 \2 \7 \9 \5 \0 \2 \8 \8 \4 \1 \9 \7 \1 \6 \9 \3 \9 \9 \3 \7 \5 \1 \0 \5 \8 \2 \0 \9 \7 \4 \9 \4 \4 \5 \9 \2 \3 \0 \7 \8 \1 \6 \4 \0 \6 \2 \8 \6  \2 \0 \8 \9 \9 \8 \6 \2 \8 \0 \3 \4 \8 \2 \5 \3 \4 \2 \1 \1 \7 \0 \6 \7 \9 \8 \2 \1 \4 \8 \0 \8 \6 \5 \1 \3 \2 \8 \2 \3 \0 \6 \6 \4 \7 \0 \9 \3 \8 \4 \4 \6 \0 \9 \5 \5 \0 \5 \8 \2 \2 \3 \1 \7 \2 \5 \3 \5 \9 \4 \0 \8 \1 \2 \8 \4 \8 \1  \1 \1 \7 \4 \5 \0 \2 \8 \4 \1 \0 \2 \7 \0 \1 \9 \3 \8 \5 \2 \1 \1 \0 \5 \5 \5 \9 \6 \4 \4 \6 \2 \2 \9 \4 \8 \9 \5 \4 \9 \3 \0 \3 \8 \1 \9 \6 \4 \4 \2 \8 \8 \1 \0 \9 \7 \5 \6 \6 \5 \9 \3 \3 \4 \4 \6 \1 \2 \8 \4 \7 \5 \6 \4 \8 \2 \3 \3  \7 \8 \6 \7 \8 \3 \1 \6 \5 \2 \7 \1 \2 \0 \1 \9 \0 \9 \1 \4 \5 \6 \4 \8 \5 \6 \6 \9 \2 \3 \4 \6 \0 \3 \4 \8 \6 \1 \0 \4 \5 \4 \3 \2 \6 \6 \4 \8 \2 \1 \3 \3 \9 \3 \6 \0 \7 \2 \6 \0 \2 \4 \9 \1 \4 \1 \2 \7 \3 \7 \2 \4 \5 \8 \7 \0 \0 \6  \6 \0 \6 \3 \1 \5 \5 \8 \8 \1 \7 \4 \8 \8 \1 \5 \2 \0 \9 \2 \0 \9 \6 \2 \8 \2 \9 \2 \5 \4 \0 \9 \1 \7 \1 \5 \3 \6 \4 \3 \6 \7 \8 \9 \2 \5 \9 \0 \3 \6 \0 \0 \1 \1 \3 \3 \0 \5 \3 \0 \5 \4 \8 \8 \2 \0 \4 \6 \6 \5 \2 \1 \3 \8 \4 \1 \4 \6  \9 \5 \1 \9 \4 \1 \5 \1 \1 \6 \0 \9 \4 \3 \3 \0 \5 \7 \2 \7 \0 \3 \6 \5 \7 \5 \9 \5 \9 \1 \9 \5 \3 \0 \9 \2 \1 \8 \6 \1 \1 \7 \3 \8 \1 \9 \3 \2 \6 \1 \1 \7 \9 \3 \1 \0 \5 \1 \1 \8 \5 \4 \8 \0 \7 \4 \4 \6 \2 \3 \7 \9 \9 \6 \2 \7 \4 \9  \5 \6 \7 \3 \5 \1 \8 \8 \5 \7 \5 \2 \7 \2 \4 \8 \9 \1 \2 \2 \7 \9 \3 \8 \1 \8 \3 \0 \1 \1 \9 \4 \9 \1}
}


\end{document}