The simplest option would be to add a "person with red hair" emoji, in which you could change the skin tone as you can currently do with a person with blonde hair, for instance. But the proposal ...

"If you say you're going to diversify, why not add a few red-haired emoji in the mix? Natural redheads may be rare at less than 2% of the world's population, but that is 138,000,000 iPhones ...