You also want to think how to get this stuff OFF the hair and skin! I would NOT use craft acrylic paint on a horse, because that stuff probably won't come off well or easily!
At 4-H, we used wet chalk sticks to "paint" the horses with various symbols, logos, or totally change their colors. Chalk did need to be washed out with shampoo, but came off nicely, no ill effects on the skin or hair of horses.
It seems that wetting the sticks of chalk gave better color adherence on the horses. Dry chalk application didn't stay on nearly as well. You might want to do some chalk painting on horses when they need a bath anyway, to give other folks the recommendations on how wet to get the chalk for best results.
I have a white paste that is designed for whitening white markings etc. I have sometimes given a pony a new star or marking with it. Your tack shop may have some. And this stuff: http://www.sabella.co.nz/ is heaps of fun.
If you can't get any, I could send some to you - it might cost a fair bit in postage to get to you in time for Halloween, but is designed specifically for horses.
Theatrical makeup can be found on ebay. It can be used on animals and humans and comes in a rainbow of colors.
Spirit gum, used to hold pasties and other things on exotic dancers, is safe for animals and holds items on skin. Not sure how it works on hair. (I learned about spirit gum many years ago when getting ready for Halloween. My then boyfriend kew about it and told me to use it. It worked.)
You can also buy spirit gum on ebay. Ebay has everything. Including costumes. One of my horses just got a nice big lion's mane on ebay so he might just be the cowardly lion this year. He's a real wuss, so it's typecasting.l
Do not use acrylics. I paint, and acrylics can cause skin issues on some people. Do not use anything on a horse or pony that you'd not use on yourself. Theatrical makeup and spirit gum have been used by humans on themselves for 100s of years. Proven safe.
Phoebetraier, white chalk to highlight white markings on horses is sold her in USA also. It is often used by QH people when showing in hand. My friends use it. Nice of you to offer. The dry kind is sold here in several colors.
Tempra paint is usually used. It can be washed off, but still may leave residual color on white and light colors.
The first time my horse was used in a horsemanship camp, they painted a yellow lightening bolt down his almost-bald face (he's a Paint). Very cute. It washed off well. The blue and red they used on his legs, not so much.
You can make a cream out of cornstarch or baby powder, cold cream, and food coloring. Make a thick paste and apply. Works great, you can make any color. Might stain white a little but it's not too bad if the colors aren't bright and it doesn't last long at all.
I know its a bit ambitious ,but Halloween is sort of a big deal hear and the dear pony gets incorporated this year we have settled on La Catrina costume for DD and a matching sugar skull mask for the dear pony complete with a lot of roses and lace.
I was thinking of blowing this up and using it as my guide.