What are some homemade recipes for paint I can put on my horse?

You do know that 'Indian' (your word, not mine,) is just a word for a group of people of a particular ethnicity, right? I'd certainly hope that you'd do some actual research to make your costume authentic and respectful of a particular tribe or culture, rather than simply slapping on a bunch of buckskin, warpaint, and feathers to fulfill a bunch of tired and offensive stereotypes..

There are paints especially made for horses (no, I'm not going to look them up for you) and there are body paints and face paints for people. Anything else is just guesswork on your part. I can already hear the comments you'll be getting around November. "Yes, her horse really is a bit pink, purple and green. She painted it last summer for the parade, made up the paint recipe herself, and some of the paint hasn't come out yet." Personally I'd try looking for horse paint and horse glitter, and then not blink at the price.

I agree with The Christ Puncher... but if you must paint your horse, do not use acrylic or poster paint on your horse. Paints such as these will tend to shrink as they dry, which is going to pull your horse's hair and also take forever to come out. Eww. There are cans of aerosol spray colors that are made for livestock. They usually come in natural colors such as white, black, and brown. There is also a few products that people use to make legs whiter in shows, these are usually chalks or liquid with a sponge applicator. You can get pretty creative with these even though they are more natural colors. Example: I made a cardboard stencil of a skull and crossbones and sprayed it on my horse's butt for a Halloween costume contest. (I was a pirate and she was the ship.) I totally won, so use your imagination.

I wouldn't advise paint, if she turns and uses her mouth to scratch an itch she may ingest some. Use food colouring.