Check out this site - I use it all the time. You click the radio buttons to select text or background, then click the colors and it shows you exactly what it'll look like putting, say, green on yellow (or whatever). Then it gives you the RGB #'s you need to use. You can also mess around with the text, change it to whatever size and font you want.
http://www.zspc.com/color/index-e.html
I think it's in Japan, but always loads fine for me.