Basics are:
lighter color on top, darker on the bottom (psychology states, that we expect to see shadows, and therefore see a darker colour as 'down')

black & white turn into specs / dissapear in the air, and you have no orinetation

same with Blue; sky / baby blue especially. Some camo shemes as well

Red and yellow are quite visible, and are probably your best bet

Put a large shape on one wing only, then you know which wing is which, and therefore know which is high, whether the plane is comming towards / away...

different schemes top and bottom also help with orintation..