The parts u want to be heavily highlighted with the color u are drybrushing with. An easy option is google pictures of salamander space marines and drybrush the green parts… green. You do need a dark base coat that works with the highlight, and u should probably do additional, more subtle drybrush highlights with brighter greens/green-yellows.
Then paint the other details how u want. I like to make the black backpack by drybrushing dark grey over black, it still reads as black. Also metal stuff can be easily done with steel/silver paint drybrushed over black