Load the xcolor package in the document preamble (\usepackage{xcolor}).
Inside a math inset, start by typing \colorbox and then press the space bar.
Now type \{, which should create a pair of braces surrounding an inset, and
type the background color name (e.g., cyan) inside. The color name will look
like a math formula (c times y times a times n), but don't worry about that.
Just after the closing brace, type \{ again, and type the actual formula in
the new inset.Paul
