jhice: As you can see in the "inline help", it's expects a "rgb(r,g,b)" formulation as well that you can/should provide.
r, g, b are also expected number values that you can provide through the use of the "int()" keyword to cast the value to integer.
For now I can't edit the FAQ anyway (because of some quirks in the new forum).
As soon as I can, I guess I'll add it, but I'm still unsure this is really something required nor frequently asked (especially since it's PHP, so out of C2's scope).
It can still find its place in the "Misc questions" I guess.