Gel Coat Cracks below waterline

[Bill Vanguard 281]

Hello folks I have removed all the bottom paint from my fine vessel planning to fix a few blisters and barrier coat.
When I removed the paint and got down gel coat at first it was a beautiful white. But within an hour or so some of the areas around the crazed areas had yellowed. Any idea what is going on here?

I plan to grind and fill any blisters/major cracks but otherwise I was going to barrirer coat with interprotect and call it good.

Thanks Bill