As long as Mahz is able to code BBCode as a direct analogue to CSS, which I think he is, it should be simple enough to utilize the basic CSS property for color. It doesn't matter if your CSS says:
[quote]color: blue;[/quote]
or 
[quote]color: rgb(0, 0, 255);[/quote]
or 
[quote]color: #0000FF;[/quote]

It'll all show up the same. Likewise, whatever BBCode format is devised, it'll likely be capable of the same range of inputs.