Tikzmath: use array

Accessing the array from inside \pgfmathsetmacro, or \pgfmathparse, seems to do what you want:

  \documentclass{report}
  \usepackage{tikz}

  \begin{document}

  \begin{tikzpicture}
    \def\n{5}
    \def\myarray{{42,1,0,1,1,0}}
    \foreach \s in {1,...,\n}
    {
      \pgfmathsetmacro\ccolor{ifthenelse(\myarray[\s]==1, "green!30", "blue!30")}
      \node[draw,circle,fill=\ccolor] at ({90+360/\n * (\s-1)}:2cm) {$\s$};
    }
  \end{tikzpicture}
  \end{document}

Note that you don't need \usetikzlibrary{math}.

The output is the same as in marmot's answer:

Inside \tikzmath you cannot do assignments except of the allowed kind (that is, to local variables).

Here's a solution, with a reimplementation of arrays and a new keyword for \tikzmath that allows executing code at the same level (not in a group). Beware of not doing assignments that change the \tikzmath state; also no “tikzmath” is available inside it, so there must be an indirection as shown.

\documentclass{report}
\usepackage{xparse}
\usepackage{tikz}
\usetikzlibrary{math}

% define a new keyword
\makeatletter
\def\tikz@math@process@keyword@assign{%
  \tikz@math@collecttosemicolon{\tikz@math@process@keyword@@assign}%
}
\def\tikz@math@process@keyword@@assign{%
  \tikz@math@collected\tikz@math@parse
}
\makeatother


%% Let's reimplement TikZ arrays
\ExplSyntaxOn
\cs_new:Npn \shp #1 { \prop_show:c { l_tobias_array_#1_prop } }
\NewDocumentCommand{\definearray}{mO{}}
 {% #1 = array name, #2 = items
  \prop_clear_new:c { l_tobias_array_#1_prop }
  \int_step_inline:nnnn { 0 } { 1 } { \clist_count:n { #2 } - 1 }
   {
    \prop_put:cnx { l_tobias_array_#1_prop } { ##1 } { \clist_item:nn { #2 } { ##1 + 1 } }
   }
 }
\NewDocumentCommand{\setarrayitem}{mmm}
 {% #1 = array name, #2 = index, #3 = value
  \prop_put:cff { l_tobias_array_#1_prop } { #2 } { #3 }
 }
\NewExpandableDocumentCommand{\arrayitem}{mm}
 {
  \prop_item:cf { l_tobias_array_#1_prop } { #2 }
 }
\cs_generate_variant:Nn \prop_put:Nnn { cff }
\cs_generate_variant:Nn \prop_item:Nn { cf }
\ExplSyntaxOff

\begin{document}    

%%% FIRST METHOD
\begin{tikzpicture}
  \definearray{myarray}
  \tikzmath{
    \n = 5;
    % Works, but one problem: how to manually choose the values
    % without using a loop?
    int \i;
    for \i in {1,...,\n}{
      \tmp = mod(\i,2);
      assign \setarrayitem{myarray}{\i}{\tmp};
    };
    % Display
    for \i in {1,...,\n}{
      if \arrayitem{myarray}{\i} == 1 then { \ccolor = "green!30";}
      else { \ccolor = "blue!30"; };
      {\node[draw,circle,fill=\ccolor] at ({90+360/\n * (\i-1)}:2cm) {$\i$};};
    };
  }
\end{tikzpicture}

%%% SECOND METHOD
\begin{tikzpicture}
  \def\n{5}
  \definearray{myarray}[42,1,0,1,1,0]
  \foreach \s in {1,...,\n}
  {
   \tikzmath{ if \arrayitem{myarray}{\s} == 1 then { \ccolor = "green!30"; }
      else { \ccolor = "blue!30";};
    }
    \node[draw,circle,fill=\ccolor] at ({90+360/\n * (\s-1)}:2cm) {$\s$};
  }
\end{tikzpicture}
\end{document}

Now it's a bit more like an answer. I was using this answer. It is a bit odd that one needs to call \pgfmathparse in an tikzmath environment.

\documentclass{report}
\usepackage{tikz}
\usetikzlibrary{math}
\begin{document}    
\def\myarray{{42,1,0,2,1,0}}

\begin{tikzpicture}
  \tikzmath{
    \n = 5;
    % Works, but one problem: how to manually choose the values
    % without using a loop?
    int \i;
    for \i in {1,...,\n}{{\pgfmathparse{\myarray[\i]}\xdef\mynum{\pgfmathresult}};
      if \mynum == 1 then { \ccolor = "green!30";}
      else { \ccolor = "blue!30"; };
      {\node[draw,circle,fill=\ccolor] at ({90+360/\n * (\i-1)}:2cm) {$\i$};};
    };
  }
\end{tikzpicture}

\end{document}