Prevent some font color/size tags

I would like my visitors to have the possibility to use different font tags like size and color in there posts but preferably within limits. It's a bit easy for some people to mess upp threads with big fonts or colors that others hardly can read, so I have currently disabled that function until I have solved the problem.

First I thought it was easily done with a simple ereg and regexp but in fact, I don't even know where to put the code if I had it.
I have search all over at vbulletin.com (and here as well) but it seems like this is not a general problem or maybe I didn't use the right terms.

If this has been issued before, please redirect me to the page or else, if anyone can give me some advace how to approach this.

FireFly: Below is hidden text:
[COLOR={secondaltcolor}]For example, this text is hidden[/COLOR]