Hahahaha toadflax you should have a go at it!! If you put the photo of the animal up on photoshop or similar, and enhance the colour you'll be able to see all the different colours that are in the coat ;) I've done so many now that I can just have a look at the photo and see what I need for the undercoats. Some bays you need to lay a bright red under, others a blue, all depends on the lighting and tone of the coat.
Liz, as for the grey, usually I like them on a light/medium purple, or a blue of some sort. In your head a grey will look superb on black, but I've made that mistake as I said above, and it looked so flat no matter what I did to it. You want to put it on a colour that is part of the undercoat colour (sorry I'm just adding things in as I think of them haha so no particular order!!) to bring it out in the final layers. So say you're doing a chestnut, I use a HEAP of blue in the darker areas of a chestnut, so my paper colour of choice is usually a blue. Means I don't have to work as hard on the base coat too because the paper colour comes through ;)
You said you used alot of yellow to highlight white manes? In coloured pencil you can do that, but if you want to give a really clean crisp feel to a white mane, stick with blues and purples ;)
Charcoal paper you can get away with, but you might struggle to layer the pastel as the teeth on the paper won't hold as much pigment. That's why I prefer to use the thicker pastel paper with quite heavy teeth on the surface - I layer ALOT! The drawing is literally off the page, you can see a ridge on the side of it :S
Look forward to seeing the next installment :)