You know how I said this would be positively the last part of this series? Well. It sort of is. This is the last of the 'Mike pontificates a lot' posts. I am systematically working on painting, and photographing stage by stage as I go, examples of most of the main colours and markings, and these should appear one at a time over the next week or two. For once, I'm not in a hurry and trying to batch paint horses by the dozen, so do actually have time to do this.
Anyway, that aside, let's crack on with some of the other relatively common horse colours. Unfortunately, these do tend to produce more variations within an expression of a gene than the 'straightforward' ones, so I'm going to be saying 'use a reference photo' a lot. Hopefully, though, the previous articles will have meant that you understand why horses are the colour they are, and that can inform how you approach painting them.
Here's where the fun starts - a quick Google image search should make it pretty clear that the range can vary from quite a dark grey (usually on a horse that started out black as a foal), through a range of shades, with our without dappling, to almost white, and past there to flea-bitten. Start by deciding what colour the animal started out (all together now: "mostly bay"), as that'll govern some of your approach, and also the mane and tail colour - if it's a bay or black, odds are the mane and tail may stay pretty dark. If the animal's at an early stage of grey, it's probably worth starting out painting it the underlying colour, and then adding the grey by a combination of drybrushing and stippling. If it's a definite grey, you can probably get away with starting with a suitable grey shade.
Really, your best bet is to pick a suitable reference picture, and work from there - greying is a pretty idiosyncratic thing! Greys do tend to be 'hero horses', unless you're working with an 1815 British army, so you probably won't be painting too many. It's also worth noting that the mane and tail on a grey tend to go at a completely different rate to the coat, so it's quite possible, for example, to have a grey with a flaxen mane!
Dapple and flea-bitten
I've had success with these with a stippling approach. For a dapple grey, start by painting the horse a darker shade (I've mixed Astronomican Grey and Chaos Black, or you could even start out with a greying bay/chestnut). Then take a large, knackered, round brush with spread bristles (the one you left in the dip too long is a good bet), pick up some pale grey or white on just the tips of the hairs, and dab it almost dry on kitchen roll - dab, rather than wipe, seems to work better. Once you can tap the end on a surface and get a pattern of tiny dots, you're in the right ballpark. Apply carefully to the horse (again, I really recommend a reference photo) with the same light, dabbing motion.
For flea-bitten (and by extension bloody-shouldered) greys, start with an almost-white grey horse, and do the same, but stippling one of the shades you'd use for bay, if anything even more sparingly.
On one of my 'hero' horses for my El Cid army, I tried the other approach - that of spraying very lightly from a distance of about 2' with Army Painter spray white. It's a bit nerve-wracking, but it does work!
As I'm sure you remember, dun is a colour dilution gene. As far as painting goes, you can treat the animal like a bay, chestnut or black, but picking the right base colour - the brown/red shade in bays and chestnuts lightens to anything from almost cream to almost pink, via some very nice honey/gold shades. Again, a reference photo will help, but the important thing to remember with duns are the 'dun face' (generally darker) and the primitive markings - a black line down the spine from mane to tail is almost always visible, and some have barring around the knee/hock joint.
Roans differ from duns in that the apparent lightening of colour is caused by an even mix of normal coloured hairs and white. The result tends to look almost washed out, if you check some of the reference photos, and I went on a hunch that a good approach would be to paint a normal coloured horse, and then apply a very very dilute wash of either white or Astronomican Grey. It seems to work.