Parkerising is a magnesium phosphate finish applied to steel. Annodising is an electrolsys (is that the right term guys?) done to aluminum. It uses electricity in an acid bath with the part being done the negative end, or annode. It actually changes the composition of the surface of the metal making it harder. The coloring is basically a dye that soaks into the pores of the metal and is then sealed.
The receivers are generally done in military hard annodised type-3 which is harder, but more expensive than the common type 2.
Yours looks pretty purple though, but it isn't uncommon for black anno'd aluminum to have some purple tint to it. Even the Colt's and FN's made for the military sometimes have that purple tinge to them.
If you don't like it, go ahead and change it. It'll be cheaper than having it redone, unless you use some sort of do it yourself coloring or spraypaint.