you kind of have to paint the turret and the tank of grav-tanks upart. I dont have mass troops in my eldar army, but in my tyranids army i do this: lay out all your little buggers on a box and paint them a light color like bleached bone, preferably using a spray can. Affter the bugs dry, dust over them using a darker color like chaos black in a can. I painted all of my tyranids in about 10 min total, over a period of like 3 hours. they look sic cool though. (wait i did paint teethe and eyes and talons and stuff other colors too.) so it was like an hour of my life whasted.