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.