FCHighlight[exp, {{symbol1, color1}, {symbol2, color2}, ...}] highlights the given set of symbols in the output using Style and the provided colors. This works only in the frontend and alters the input expression in such a way, that it cannot be processed further (because of the introduced Style heads).
FCHighlight[Expand[(a + b + c)^2], {{a, Red}, {b, Blue}}]2 a b+2 a c+a^2+2 b c+b^2+c^2