Painting airplanes, what comes first, bottomside or topside?

Take your pick.  Although the paint dark over light is a good rule of thumb, there are other considerations.  If you're doing camo and don't want to leave your tape down for too long, it's easier to first do the camo on top and then do the underside.  I've done both ways and don't have a huge preference one way or the other.

lets take for example an SEA scheme.  say you have a Phantom with typical SEA on top and light compass grey belly .  Paint the belly with your ligth compass grey to the demarcation line, and then lightly overspray on top with the grey. This acts as a final flaw-checker and preps the surface for paint to come.

Next comes the Dark Tan, medium green and finally dark green.  I would normally seal the kit at this point with a layer of your favorite clear medium, and proceed on to painting the antennas, leading edges, etc and seal that.

HTH

David