There is no color that can be added to green to make it purple, all you can do is make the green darker, anywhere from olive to black at this point, problem is that you can't remove the yellow which is what was mixed with blue to make it green and you can only add red which will darken to a shade of olive.
*** Side Note ***
Not sure if anyone here was ever on URC, but there was a famous thread over there called "Shadow Slash" where a guy was trying to sell his Slash which he claimed that he dyed it black but you can clearly see overspray from spray paint on the shock springs... the comments on the thread were hilarious, not just the bad spray paint job but a long list of things wrong with the poorly maintained Slash, the OP would persist to claim it was dyed.... I can't give the thread justice but if anyone has any screen shots from it, it would be worth sharing, ha!