To obtain different color schemes for each aircraft simply find the hidden pilot 
within each level. The pilot usually stays a couple of grid squares away from the 
main area of combat. Fly around the main area of battle and until you run into 
the hidden pilot and shoot him down. Sometimes you have to travel far distances 
to find the lone pilot. You will know the plane is the hidden pilot for that level 
when the aircraft model and the pilot's name appear on your reticle and when a 
lone orange plane appears on your radar. Note: S-37's also appear orange on radar. 
Repeat this process until you have found all color schemes for each aircraft. 
In order to find all color schemes you must beat the game 3 times to unlock the 
different aircraft. There are two optional color schemes for each aircraft, which 
also cost more than the default aircraft. There is no optional color schemes for 
the X02.
Submitted by tunafi2000