How can hair change colour along its length?

Your hair is coloured because you have cells - called melanocytes - that pump pigment into the hair as it's growing.

As you get older, these pigment cells basically get a bit knackered and they stop putting colour into your hair - which is why your hair goes grey. It doesn't actually go grey, in fact, it goes colourless: it goes white. But, against the background of darker hair, it may look grey. This is because the pigment has stopped being pumped into the hair.

Also, pigment cells don't continually pump pigment into the hair. They may take little breaks, going in a cyclical way, and so it's perfectly possible for hairs to be different colours along their lengths.

It's probably unusual that you'd have zebra print hair, but it is certainly possible that they might stop producing pigment for a bit and then start producing pigment again.