You probably could patch in a closing group but you need to be careful not to interfere with the way section headings look ahead for * or optional arguments. Also putting a color command immediately before the chapter command is quite likely to affect the vertical spacing.
It is better really simply to add the colour at the point the font is added. In book class the chapter head is defined by
\def\@makechapterhead#1{%
\vspace*{50\p@}%
{\parindent \z@ \raggedright \normalfont
\ifnum \c@secnumdepth >\m@ne
\if@mainmatter
\huge\bfseries \@chapapp\space \thechapter
\par\nobreak
\vskip 20\p@
\fi
\fi
\interlinepenalty\@M
\Huge \bfseries #1\par\nobreak
\vskip 40\p@
}}
so I'd do
\def\@makechapterhead#1{%
\vspace*{50\p@}%
{\parindent \z@ \raggedright \normalfont
\ifnum \c@secnumdepth >\m@ne
\if@mainmatter
\huge\bfseries \textcolor{myColor}{\@chapapp\space \thechapter}%
\par\nobreak
\vskip 20\p@
\fi
\fi
\interlinepenalty\@M
\Huge \bfseries \textcolor{myColor}{#1}\par\nobreak
\vskip 40\p@
}}
Which avoids adding the colour in vertical mode.
I see you added a comment that you are using report so you would make the same additions to:
\def\@makechapterhead#1{%
\vspace*{50\p@}%
{\parindent \z@ \raggedright \normalfont
\ifnum \c@secnumdepth >\m@ne
\huge\bfseries \@chapapp\space \thechapter
\par\nobreak
\vskip 20\p@
\fi
\interlinepenalty\@M
\Huge \bfseries #1\par\nobreak
\vskip 40\p@
}}