\def\colorlist{red,green,blue,yellow,cyan,magenta}
\ExplSyntaxOn
\cs_new_eq:NN \ClistItem \clist_item:Nn
\ExplSyntaxOff
\let\TeXleft\left
\let\TeXright\right
\makeatletter
\newcount\leftcount
\protected\def\left{%
\global\advance\leftcount 1\relax
\begingroup
\color{\ClistItem\colorlist\leftcount}%
\TeXleft}%
\protected\def\right#1{%
\TeXright#1%
\endgroup
\global\advance\leftcount -1\relax}