It depends on how dark the grey is, if it's a light grey, almost white then that is most likely fin regrowth! That's good, that means he's healing. Also, you realize that PimaFix is meant for fungal infection not bacterial as fin rot usually is? I suggest doing some frequent water changes to his tank to keep his water nice and healthy. Usually fish only get fin rot if their water is bad, like high ammonia and stuff.
Do you have a testing kit to test your ammonia and pH levels?
The grey that is under his head is just from stress. Betta's and most other fish will lose their color when they are under stress or don't like something and you'll see a change of color. Usually they get dull so a blue Betta would almost turn brown-ish with two black stripes going down it's side, those are stress stripes which indicate what they sound like they do; stress. And then once he's feeling better, he'll color up again. But the grey underneath is very normal for most Betta's, even if they aren't much stressed.