-Well, generally Necrons are the easiest to paint.They are usually paint a metallic silver or such. A few swipes of paint, then paint eyes and maybe the chest symbol, base him and he is done.
-After that, standard Space Marines are pretty easy to paint with their big fat areas (like shoulder pads, greaves.)
-Then id say Tau. (maybe Tau before rines)
-And then Dark Eldar?
-And then maybe orks, a bunch of green and a few details.
-Then Maybe nids if you do them simply. But if you do them well they are time consuming.
-Then IG?
-And then Eldar with their fancy designs.
-And then Chaos rines with their fancy trophies and stuff.
-And then Witch Hunters.
Really--this is inaccurate. If you simply paint them efficiently and to "table-top" quality- meaning look good standing a few feet back, then thats all you need, and the armies are all the same.
I was going for a more paint-the-details-list.
You can paint any way you want. Most people do there table-top standard as three to four colors.
Use this as an example:
http://www.evil-bob.com/orba.jpg
You can see how he could have painted such a model much better, but he used a very simple scheme. Brownish on armour, one flesh color, one blade color and a few other colors to spice it up. The base is a bit of grass with a few rocks.
That model might have taken 15 minutes total.
So dont stress about being an amazing painter!
