Here are a couple of photos of a KAC M5 RAS I used to have that was bright purple.
When I was in Iraq, the FN M16A4 I carried was purple.
When aluminum is anodized, it makes a thicker layer of aluminum oxidation. Then this can be dyed different colors. Black tending to be one of the most common. Often then a sealer is applied over the dye.
At my workplace, we make plenty of parts out of 7075-T651 alumnium and have them mil-spec hard coat anodize black, much like a M16. I’ve seen all sorts of blemishes in the parts I have received back at work but I’ve never received a purple part back.
I wonder if the purple came from containment on the aluminum or in the anodizing solution. Or it might have been a bad dyeing job.
In general, it is a bad idea to have something re-anodized, as the original anodize coating grew into and out of the original metal. That would have to be removed for the refinish, and that will change tolerances of the part. It can make holes (like the trigger pin holes) over sized.