MXP allows you to colour your text or background in a very rich way, eg.
<color fore=brown back=lightcoral>
You can also use any arbitrary RGB (red, green, blue) combination.
The colour names and RGB use is described on MXP Colours.
MUSHclient also has an inbuilt "Colour Picker" (Ctrl+Alt+P) which lets you browse through all of the inbuilt names, plus select your own colour.