Ok, here's a post with tips on how I paint to make my mobs look consistently inconsistent and identifiable.
Now, most people will play a specific clan of Orkz to have their entire army look cohesive. I, personally, am including units from all the clans (even some rebel grotz).
The way I do it is to pick four colors that best represent the clan of the unit (Since most of my WAAAGH! is Deffskullz, I'll use them as an example). For this clan, I chose blue, black, brown and khaki.
Now take your average mob of 30 boyz. With 3 boxes of boyz, you can make a mob of 30 boyz or 3 mobs of 12 boyz (for trukks). The way to identify the 30-boy mob is the straps around their legs (or wraps around their ankles).
You can see that, from behind, these straps and wraps are very visible. You can also easily identify that this mob of trukk boyz are the ones with black pants (the other two pants colors are brown and khaki). The straps and wraps will match the bosspole of the Nob in color. Blue Bosspole goes with blue straps and wraps.
This method looks better than painting rings on the bases, or having all the boyz in one mob wearing helmets and another mob not wearing helmets. The boys look more random, more mob-like but are still very easy to identify and keep track of.
For the shirts, use a color that you didn't use for the pants. So black pants would get Blue, Brown and Khaki shirts. Brown pants get Blue, Black and Khaki shirts. Khaki pants get Blue, Brown and Black shirts.
This method can be transported to other clans as well.
Goffs - Black, White, Brown, Red
Badmoonz - Black, Yellow, Brown, Khaki
Evil Sunz - Red, Yellow, Black, Brown
Blood Axes - Red, Green, Black, Brown
Snakebites - Blue, Yellow, Brown, White
Honestly, you can choose whatever colors you like, but this method will allow you to keep them uniform throughout your army, giving it a sense of unity but still preventing your boyz from looking uniform (unless you're Blood Axes and want uniformity).