\documentclass[10pt]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}
\usepackage{color}
\pagestyle{empty}
\begin{document}
\definecolor{MyColour}{rgb}{0.50,0.00,0.00}
{\color{MyColour}
$ a + b = c $
}\end{document}