you want as much contrast as possible with the sky.

white is ok cause its rare to get perfect "snow" white clouds and it contrasts with the blue of the sky. (as long as you have a contrasting secondary color)

blue and purple are a bit diffrent as they can blend in with darker clouds (I have a purple and white Somthin' Extra and during an overcast day I cant make out orientation sometimes the purple blends in too well)

also black is a good color as a secondary because unless you are flying at night it contrasts with just about everything. (solid black plane could blend in with shadows while close to the ground so you need something else)

as for shapes go for a single strip, or solid on the top and several stripes (or if you are up to it) a checker pattern on the bottom, you want something that is easy to tell top from bottom

putting stripes on the tail can help with front/back orientation so don't forget about the tail surfaces

just my opinion