I vote for dye using KoolAid, pink wraps on the legs, darker (or lighter, depending on how dark the whole horse is) pink awarness ribbons stenciled on the butt, pink sparkles on the feet & butt, pink curling ribbon streamers in mane & tail & hanging from bridle, saddle, etc., maybe find pink reins or wrap your reins in pink ribbon (like the fluffy kind you use to knit hats??), a pink lei, boa, string of flowers or streamer sort of something around the neck like a necklace, plus I'm assuming YOU will be decked out in pink, too! Definitely hit the dollar store & find whatever you can.
Please, please post pics! Sounds so fun!