Signatures in Wikipedia are identifying information that you put after writing a comment on a talk page. They tell other editors who wrote the comment and when. This can be done by writing ~~~~.
Signatures can optionally be spruced up with colors. To add color to your signature, go to the Preferences link at the top of the page. In the nickname box, enter [[User:MYUSERNAME|<font color="MYCOLOR">MYUSERNAME</font>]]. Replace "MYCOLOR" with a color you like, such as "red" or "green". Replace MYUSERNAME with your username. Finally, check the "raw signature" box, and save your preferences. Now, when you type four tildes ~~~~ in a talk page, your new signature will appear.
The color names that can be used in standard HTML are given in the standards document here. Using the hexadecimal RGB-style colors (such as #008000) you can choose any color you like from about sixteen million, not just the 140 colors that have names in standard HTML.
More complex signatures are possible; however, your signature as typed into the box above should not be unnecessarily long. Long signatures make pages larger and harder to edit, and are discouraged.