Purple, that means that there are red pigments in the color, most unstable of the color pigments.
Take at least a 10X magnifier and look "very closely" at the affected surface to see what is really there.
That will give you a clue of a lot of things.
If you can get ahold of a 30X lighted magnifer, that is even better.
Diagnois is the key to understand what you are really dealing with.
It's easy for people to say, "I done this or that", but from afar, and even with pictures, there are so many variables with gelcoat, only you can look and see, up close, what is going on.
Scratches, marring, etc are one thing, but "bubbles", which can only be seen under magnifcation, color variances, etc, in the gel coat, you have to see and work from there.
Brumpy