Section title formatting also in the toc

Here is a solution. I will try to explain if needed.

\documentclass{article}
\usepackage{xcolor}

\newenvironment{myenv}%
   {\color{red}%
   \addtocontents{toc}{\protect\begin{mytocenv}}}%
   {\addtocontents{toc}{\protect\end{mytocenv}}}

\makeatletter 
\let\mtl@section\l@section
\newenvironment{mytocenv}%
   {\renewcommand*\l@section[2]{\mtl@section{{\color{red}##1}}{##2}}}%
   {}
\makeatother
\begin{document}
\tableofcontents

\section{One}
Section one.

\begin{myenv}
\section{Two}
Section two.
\end{myenv}

\section{Three}
Section three.
\end{document}

Update I did not take care of subsection and so on (well that was not explicit in the question) so I think one should do

\makeatletter 
\let\mtl@section\l@section
\let\mt@dottedtocline\@dottedtocline
\newenvironment{mytocenv}%
   {\renewcommand*\l@section[2]{\mtl@section{{\color{red}##1}}{##2}}%
   \renewcommand*\@dottedtocline[5]{\mt@dottedtocline{##1}{##2}{##3}{{\color{red}##4}}{##5}}}%
   {}
\makeatother

You could also define a command instead of an environment; makes it easier IMO if you have consecutive sections which you want colored:

\documentclass{article}
\usepackage{xcolor,titlesec,titletoc}

\newcommand{\mystyle}[1][black]{%
  \titleformat
    {\section}%
    {\normalfont\Large\bfseries\color{#1}}%
    {\thesection}%
    {1em}{}%
%
  \titlecontents
    {section}[0em]%
    {\addvspace{0.3pc}\sffamily\bfseries\filright\color{#1}}%
    {}{\hspace*{0em}}%
    {\titlerule*[0.7pc]{.}\bfseries\contentspage}%
}

\mystyle

\begin{document}
\tableofcontents

\section{One}
Section one.

\mystyle[red]
\section{Two}
Section two.

\section{Three}
Section three.

\mystyle
\section{Four}
Section four.

\end{document}