you're probably looking at caliban green as the base green, given it's probably the closest to pthalo/permanent green. i don't think anyone's done what you're asking—you'll just have to eyeball it
#the base RGB of citadel paints?
jagged swallow
you can, however, pick several traditional paint colors and see if citadel has any close matches in a paint program like paintrack or brushrage, though the colors aren't always totally right
kantor blue is also the base blue i think
violet spire
damn
zenith flame
You might find this site useful: http://www.realcolorwheel.com/colorwheel.htm. The author does a good job listing out pigments that give "pure" colors on the color wheel. Unfortunately GW and other hobby paint manufacturers don't disclose what pigments they use, so Dawn's advice is right if you want to find GW equivalents.
That said, plenty of folks use artists acrylics for minis. If you really want pure pigment colors, you could consider that path too