maybe someone who is ever more knowledgeable than me about the board software (like exoray) might know a workaround
It can be done, but off the top of my head the quickest and dirtiest 'safe' way to do it would be to only allow a few select colors with custom BB code that you know doesn't effect other skins, rather then having the forum try to sort it out from every color available... Not saying a fancier way can't be worked out just that it would require more thought process then I can toss at it right now...
Basically you keep the color BB code off like it is and implement a custom BB code set this should be an option in the admin control panel as shown below...
http://www.vbulletin.com/docs/html/bbcode_add
Add say BBcode like this
tag [RED] replacement <font color="#FF0000">{param}</font>
tag [BLUE] replacement <font color="#0000FF">{param}</font>
tag [GREEN] replacement <font color="##008000">{param}</font>
tag [YELLOW] replacement <font color="#FFFF00">{param}</font>
tag [PURPLE] replacement <font color="#800080">{param}</font>
tag [ORANGE] replacement <font color="#FFA500">{param}</font>
tag [LIME] replacement <font color="#00FF00">{param}</font>
tag [PINK] replacement <font color="#FFC0CB">{param}</font>
tag [BROWN] replacement <font color="#A52A2A">{param}</font>
{{ I used the color chart here for common color names vs HEX number
http://www.w3schools.com/HTML/html_colornames.asp }}
Simply add code like the above via the control panel, make buttons (as font colored buttons are probably a good idea to make it simple for the members vs remembering all the BB codes) the buttons can just be an appropriate colored letter or a letter with the font colored bar under it similar to the original vBulletin font color selection tool...
This limits the available font colors to a set pallet that doesn't conflict with skins/themes, while allowing some use of colors by the members...
**As I have said before at this current time I do not administer any vBulletin forums so I can't test or double check what I just posted to verify if it will work or not, just winging it based on memory and past experience right now...