You can use any color that has a name in html on the boards and it will work.
However, since the introduction of ENWorld skins, it's a bit iffy to do so anymore. Lots of people use colors that work great on the classic look of the boards but which I can't read on Stealth, for example. I had to check my sig file on all the skins to make sure it was readable, and then change to just red for extra highlight and blue to match my avatar. Although even then, I had to tone the color down to work on the darker skins.
Here's a pretty good link on named colors in html:
http://www.lightlink.com/xine/bells/namedcolors.html
Linear Mode
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