It sounds like what you are desrcibing is highlighting, where sharp edges stand out as they catch the light.
This isn't exactly the best quality photo as I had a poor camera when I took it, however, you can plainly see the highlighting I did on the yellow panels and the blue edging on the black surface edges.
Everybody finds their own style of highlighting vehicles. I personally paint as neat an edge as possible in the chosen highlight colour then tidy up with the next colour down. Others prefer drybrushing and a technique called edging where you draw the brush over an edge. Try a search of this site for further painting advice, or Google 'model vehicle highlighting techniques'.