Because it’s my little cousins birthday and as it’s a lockdown there isn’t much we can do so we was going to colour her mane and tail and we brought a unicorn horn so she can ride a unicorn(she wanted an all pink one but I limit at mane and tail)??
Maybe hair chalk? I would just let her do the tail and maybe stencil 'cutie marks' on the hindquarters as it shampoos out and you should keep away from the horse's eyes.