Stage 1
Undercoated in white. I'm currently using Plasti-Kote Projekt Paint - Primer Super.
Stage 2
I painted the body a solid colour using Winsor & Newton Neutral Grey. I don't do the mane and tail at this stage. The paint is not thinned noticeably - I use it more or less as it comes out of the tube, with the brush well wetted to help it mix and flow. It still took two coats to get a smooth finish on parts of the horses.
Stage 3
Winsor & Newton Payne's Grey gets slapped on the areas that are going to be darker grey. This includes the recesses and the areas of the horse that'll be dark when I'm done - the rump and the muzzle, for instance. I'm using enough water to get the paint fairly runny, and dilute a bit more to give the mane and tail a wash. You can see from the photo of the Amazon horse that the paint is still wet. Best to let this slapdash layer dry before you do any serious blending. I put a wash on the hooves at this point, too.