There are lots of ways. Do you want to "shade" the panels which creates a darkened panel line? Or do you just want to add some color to the panel lines?
After sealing the model with some kind of gloss coat you can use cappilary action with artist oil paints (burnt umber for example) thinned with naptha (lighter fluid). Just add a drop every couple inches and it will seep along the panel lines. Wipe up the spill over wtih a qtip.
Shading panels is a better effect IMO but is more involved.