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 "my 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 at this or this external link. Using the hexadecimal RGB-style colors (such as #008000) you can choose any color you like from about sixteen million, not just the sixteen 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.