They were beige, darkening to orange on the thickest parts of the costume (which were generally the raised parts - ridges, nozzles etc). Many shots of them, particularly inside their spaceship, were shot in green light.
I'd start with VMC Iraqi sand and VMC orange-brown, with a brighter orange on the most raised parts.
Modified: Here's one of mine, reposted from my
Doctor Who 2010 thread: