Well, this looks like a topic worth a minute or so of my time.
By default, I use a light hull. I'm so terrible at maneuvering I need all the advantage I can get. Now, I stack it full of boosters (they're practically extra lives, y'all know), grenades (the better to kill you with, my dear), and generally missiles too. I normally waste at least one missile at a distance and grab an extra grenade at the first opportunity, so really I ride a light hull with 7 grenades, 2 missiles, and 3 boosters.
Exceptions: for Choke, I like to use medium hull; the tightness of the playing space means that maneuverability is less important. I also use a medium in Showdown, since the goodie with the boosters in it seems to have been placed with a medium's jumping height in mind. I've experimented with a heavy in Baghdad, but light is still best. For Arsenic I prefer a light, but if my partner(s) is(are) in light(s) I sometimes take a heavy. For A Bridge Too Far, I use a heavy; the ammo is too hard to reach. For racing-type levels (Avarathon, etc.), I use an unarmed light. For Wall Door World Arena, I select a heavy, even though all the hulls are identical (this prevents me from accidentally taking only 6 grenades instead of the maximum). If my aim is worse than usual, I'll use a medium on the AA levels (Bwadi, etc.), in order to get those precious two extra grenades.
Color: I like both white (looks coolest) and black (good camouflage on many levels); however, it's not worth the trouble of remembering to switch back to normal colors for team games. Therefore, I prefer green; it is the color of nature. That being taken, I'll go with purple (the color of royalty) or cyan (another color of nature). I don't actively avoid red (the color of violence) and yellow (the color of sickness), even though they're substandard. Magenta is associated with femininity; if I get stuck with it I generally turn white. I'll also turn black or white (whichever color other players are using) if I feel that I'm getting ganged up on.