All Abrams come with a set NATO 3-color pattern. More recently, some newer tanks are painted all sand or NATO 3-color based on the destination of the tank. Having processed battalions worth of brand new Abrams tanks, I can verify that the scheme is identical.
Even before vehicles came in factory painted schemes, we had set patterns that units painted. To get an idea of how this was done, think of a large piece of your average construction equipment like a bulldozer or bucket loader. Stand there with a copy of a camouflage scheme a little bigger than your average model instruction sheet.
If you are fortunate, you have a working paint gun and paint the entire vehicle the base color. Let's say forest green. Now have a couple of guys crawling around with chalk (yeah, like school teachers use) mark off the scheme on this large piece of equipment.
Make sure they label the various areas with abbreviations like BL for black, BR for brown and G for green. Now grab a couple of wide brushes like you'd use to paint your house. Start painting the brown and black spots by hand. Do this all day and into the night until you are finished.
Thankfully they now come with the factory scheme.