God damn it, I'm feeling old. But yes, that's not a good idea unless your guns are made of mithril. All the cool kids used to go through a lot of Boltgun Metal, in fact you can't paint necrons without it. Chainmail used to turn into it when you mixed it with black, not sure how the new stuff would work.
Drybrushing is well is kind of tricky, so for your level just use a perfectly dry brush (get it?), dip the tip in the colour you've mixed, and wipe pretty much all the paint off of it. Then just lightly dust the area, this should catch the raised areas. More pressure means more gets applied. Don't clean the brush in water until you're done with that colour, then don't use the wet brush for drybrushing until it's... dry.
This is pretty much the easiest way to get cheap highlights. If you've got the time, sort of paint it in slight layers of increasing amounts of white, with the highest layer being anywhere from 50 to 70 percent white. I don't know the right amounts for the colours you're using, so mix it up slowly until you feel it's right.
Another thing, the main reason to thin your paints is to paint many thin layers. Plenty of new painters thin their paints, but slather them on anyways. Don't just paint flat colours, but since that takes a bit of time, everyone just washes anyways.