For signatures you can change the colors the same way you can change the colors of your posts.To change the color of your post just write your message. and then click the button that shows small squares of different colors. It should look like this in your message.
[color=blue*]Color [*/color] Just with out the * and end up looking like this color
You can do this with many colors. just put in the name of the color after the = . I hope that helps.