The trick with an angle grinder is not to try to cut the metal with one pass. That way, you are wasting the wheel and effectively heating up the metal, causing distortion and discoloration.
The best way to cut with an angle grinder is to do so in multiple passes, and using the bottom part of the wheel to cut.
For example, if you are cutting a straight cut on a flat sheet of metal, say 6" long, place the angle grinder on your cutting line, and let the weight of your hand only carry the grinder forward those 6". After that, lift the grinder, and go back to the beginning and use that "track" you just built as your guide, and slowly keep cutting it that way.
This way, you will use much less off your wheel, (which you will notice by the reduced smell and dust in the air), you will cut with minimal discoloration, minimal heat, which will also extremely minimize distortion on thinner gauges metals.
If you still have minimal discoloration you need removed, you can use things such as Zep Polish or a Scotch Brite pad, specially on #4 finish stainless, for example.