You can use color codes on signs to color sign text. You can find the color codes here: Minecraft color codes.

Example:

&eYellow text
&aGreen&0Black
&4Red
&9Indigo

To use colors on signs users have to have the following permission: magicsigns.color

From MagicSigns 1.4.0 and above Formatting codes are also allowed, just type &n to have the following text underlined.