The coloured ones are two part. The black keycaps (which came with the keyboard originally, I didn't get them from Unicomp) are single part. Unicomp doesn't make black printed keycaps (they will sell you blank ones though) because the pad printing process they used with the black keycaps failed miserably. The picture I have there is quite flattering, the keycaps are in quite poor condition if you saw them in person.
IBM, Lexmark and Unicomp made/make custom keycaps for various keyboards. Unicomp has a wide selection of coloured keycaps for their custom stuff. I'm not sure if I have ever seen any IBM Model Ms with purple/lavender keys.
I'm jealous - how much did the keycaps cost in total if you don't mind me asking, because if I could make my Model M look like that I would be seriously happy.
They are $2 each for printed keycaps, with a minimum order of $10. I can't remember what the blank ones cost, they're cheaper than the printed ones though.