Highlight the text that you want to color. In the tools above the text field, click on the drop-down arrow to the right of the A with a dark line under it. This will open a menu of various color choices. Click on the one you want and the text should change color. Make sure you click the "Save Signature" button at the bottom when you are done.
If the extra tools are not available when you post/in the signature editing area, you may have to do this first:
Go to your User CP
On the left hand menu choose "Edit Options"
Scroll to the last item on the page "Miscellaneous Options"
In the drop-down menu, choose either "Standard Editor" or "Enhanced Interface"