How do I know what colour to use when Drybrushing?

you generally use a lighter shade of the color you are going over. you can use a light gray or silber to go over flat black like an aircraft cockpit.

I dry brush for chipping and wear,so I use the underlying color,primer or bare metal mostly.

Darker paints work as well.  I have drybrushed exhaust and dark oil stains.

Dry brushing puts down a translucent, very thin tint of the color you use, and you build it up gradually.  Thus, if a color is the wrong shade, just stop as soon as you notice it is the wrong color, and try another color.