You could do a kandy koncentrate mixture in with some SG 100 or some clear. Not sure if you can mix the actual UKs with anything else. If I had to guess from the way you explained it, you probably had a little more overlap and or moved a little slower on the rear fender than the other parts. Thats just my 2 cents. You can go back and try to darken up the other parts but theres the chance that say 2 coats will be too light, and when you add the 3rd it'll be too dark. Very touchy that way, thats why it's best to get hem all sprayed at the same time. Good luck to you. It can be done, I've done it with Cobalt Blue kandy before.
Scott
Scott