Same with me: number form synesthesia. It’s especially useful because I write historical fantasy and can see how any even fits with other historical events.
Agreed. I get optical migraines, and mine start with a small crescent shape of blinking lights. Gradually the crescent gets larger until it fills my field of vision. People experience optical migraines in different ways, but I doubt many if any see colors around people and only people.
What I CAN tell you is that migraine aura has been a sort of catch-all when docs can’t explain a visual phenomenon. The vision Paul of Tarsus (allegedly) had on the road to Damascus? Migraine aura. (Maybe the fact it’s called an “aura” is the problem.)The Cheshire cat in Alice in Wonderland? Migraine aura. Dodgson (Lewis Carroll) DID get migraines with aura, but it’s been used to explain virtually everything in Wonderland and Through the Looking Glass. He wasn’t that creative: it was just migraine aura! 
I don’t know what causes people to see auras. I wouldn’t rule out synesthesia, but it may be various conditions or none at all.