This is normal, assuming what you're talking about is like the magazine pictured on Tantal's:
http://tantal.kalashnikov.guns.ru/akbstparts.html
Scroll down until you find a picture of four AK-74 magazines side-by-side and you'll see a plum with both a black and plum finish. I have a mag that's exactly like that one and if you look at the mold number (26), mine has the same exact mold number. Check and see if your mag has the same mold number.
http://tantal.kalashnikov.guns.ru/akbstparts.html
Scroll down until you find a picture of four AK-74 magazines side-by-side and you'll see a plum with both a black and plum finish. I have a mag that's exactly like that one and if you look at the mold number (26), mine has the same exact mold number. Check and see if your mag has the same mold number.