There are home Parkerizing kits(literally stove top). The colour is changed by changing the chemicals. Here's a how-to. http://www.jouster.com/articles30m1/parkerizing.html
Mind you, if your mags are aluminium, you need to anodize or use Birchwood-Casey's Al Black(works like cold bluing). BBQ paint will do for Al too. Parkerizing doesn't work on Al.
Duracoat is an option too.
If your talking about the aluminum dri lube coating check out Armalite's website in the library section. Scroll down until you find the article on magazine finishes. There is a spray paint which matches the dri lube finish (grey in color).