Why does the roof of my mouth look purple?

Food coloration: From pigmented food or drink are one possible explanation . It could be a bruise if there has been recent trauma to the area. If it persists more than 2 weeks have it examined by a dentist.

Purple palate?: Did you eat or suck on something purple? If yes, that's why. If you didn't, I have no idea and if it persists you should see your general dentist or an oral surgeon for evaluation to determine cause.

Purple roof of mouth: Given your age an no hx of systemic problems it could also be a bruise. I would have it checked out by your dentist.