There is no such thing as "Mastering w/ white and black"
1) You can't fully 'master' one color and play it perfectly. In fact, nobody, not even the world champion can play " 100 % perfect" chess in general.The best chess engines may come infinately close but even they aren't 100 % perfect.
2) You should be playing as both white and black in games.Never only white or only black.
The idea is to improve in general in chess. It shouldn't matter as much whether you are white or black. Generally, as white you should always be aiming for an advantage while as Black you can choose to equalize and then go for an advantage or go for an advantage straight away (which is harder and more risky too)
Of course, at our level it doesn't matter as much whether we play white or black but most people find playing as white a bit easier.
Focus on increasing your tactical ability and your positional/strategical skills and learn a few openings. Don't play system openings like some people do though since it doesn't get you exposed to different pawn structures/ideas and a variety of tactics, etc...
I've played chess for almost a year and I've haven't heard any advice from the few books I've read or this forum on some insights about that question. I'd love to hear from people 1800+ but intelligent comments from anyone are greatly appreciated!