Sadly, the camera is more than a bit unforgiving with the flash and light tent, but here's a quick stage-by-stage.
First up, undercoat in black, and then undercoat those bits of the horse which are going to be white in AP Barbarian Flesh, the colour of the underlying skin. There's an argument that this isn't strictly necessary, since the AP White covers so well, but I find it focuses my thinking on what the horse is going to look at good and early, and you do need some flesh colouring.
Next, paint the areas of the horse that should be brown with a suitable shade - as I was going for a reddish bay, I used up some of my Citadel Dark Flesh, but in general, use a reference photo and mix a suitable shade. As its a bay, it should have a black mane, tail, muzzle and legs, so leave the mane and tail black and fade the brown where it meets the legs and muzzle.
|Yeah. I missed a bit or two. But you|
get the idea.
Next up, paint the white areas - I used AP Matt White, which covers brilliantly. The muzzle, if the white extends all the way to the mouth, will once again be skin coloured, but under white, that's flesh shade, so feather the white into the flesh tone.
The hooves of white-socked horses tend to be a shade of khaki, so I used Vallejo Khaki - if you're sticking to AP, a darkened Necrotic Flesh or Skeleton Bone should do the job.
The saddle blanket is AP Crystal Blue (I think - it was a week ago!), and the saddle pad is Skeleton Bone (most of it is going to be hidden by the saddle moulded onto the rider, so I'm only suggesting it. Harness is Vallejo Red Leather, which I use largely because I never paint a horse that colour, so it'll always show up against the horse.
If you can be bothered, drybrush the mane and tail with a very dark grey or brown.
Finally, the neat bit. AP Soft Tone ink wash on the white bits and khaki hooves, which will stop the colours being quite so stark as well as picking out the shadow detail, and AP Dark or Strong Tone (depending on the shade of brown you chose for the horse's coat) on the rest. And Bob's your uncle. In an ideal world, you probably want to see if you can find a slightly satin, rather than completely matt, varnish to finish the job.
And yes, I'm sure you can do better than me, but I hope you get the idea. If you're batch painting 72 of the little sods, the process, or something based on it, will get you a convincing looking (from any distance that matters for wargaming) bay horse in not that long. One tip - if you're batch painting, the maths works out roughly that you should pick out your hero horses that are going to be fancy colours, then split about a quarter of the rest off and earmark them for chestnuts, split a quarter of the leftovers from those off and earmark them as blacks, and paint the rest (which should be a bit over half) bay. The further back in history you go, the fewer chestnuts you'll probably have. I tend to work with a palette with a range of browns on it, everything from a reddish brown (GW Dark Flesh) through to an almost black brown, and will quite often do a bit of subtle mixing every few horses. But above all? Use a reference photo or two for colours, and vary them.