Yellow and Black or Yellow and any other of the DARK colors work well. fluorescent green is the most visible color but is an unstable color in sun light IE fades.

Second that. Piano keys 4" wide are nice. A strip of similar size on top close to the wing tips helps detect banking. The Apprentice is my favorite electric trainer but I would ask you to consider a Sig LT 40 Kadet, it has a 72 inch wing span and can still "get little" in a HURRY!