These are things of beauty!
Humbrol isn't listed for ROTJ gaunts at the link below (Humbrol is listed for some other Fett parts), but a combo of Floquil and Testors is. It might provide a good starting point and you may be able to find some equivalency charts to get you close. MindMeister put this together last year and others then contributed, but in its essence it is based on some of the awesome paint tutorials on the forum. Check it out!
Boba Fett Paint Colors - MindMeister Mind Map