My absolute favorite color to paint is, by and far, BLUE.
There are more shades of blue than any other color. By far. It can give you more options, more degrees of variance, and presents a fresh, unified appearance. It can be dark and foreboding or bright and vibrant. It truly gives you a plethora of options!
I'd have to say my second choices would be white and green. White is really easy to get right, it just needs some layering. Green can pop on the tabletop like nobody's business.
My least favorites would be Orange and black. I find both of these difficult to layer up with smooth gradients. Not impossible, it just takes me longer for some reason. Never quite satisfied with the results.
Thus far, my army colors are:
Empire (my largest): Blue and White
Wood Elves: Blue and Brown
High Elves: Blue and Gold
Dark Elves: Purple and Black
Lizardmen: a mix of bright colors! I try to give each unit a unique color
Ogres: Blue (skin and everything - Ice Ogres theme)
O&G: Green (obviously) and Black
Skaven: Black and Brown
Beastmen: Green and Brown
Tomb Kings: Bone (obviously), Teal, Blue and Yellow
Vampire counts: Black, Purple and Red
Adeptus Mechanicus: Blue and Brown
I eventually plan on starting a Bretonnian army as well, but I don't really feel all that pressured to do it since I'm pretty well covered on options. But when I do, I'll probably go with a multi-colored scheme with each knight representing their own estates and houses. My general and main bulk of the army will be Black and white/silver. My ancestry can be traced back to knight heraldry with a black and white checkerboard pattern (predominantly black)with a white and silver crescent emblem in the middle. Gotta represent my house!
Green. Green is the best color. There is no color more gooder than green. My Empire army is Stirland. I've got night goblins. And Nurgle. I also have a space marine chapter that is green. The second best color is red. Blood for the blood god and all. Also space vampires.