After lots of research I painted mine bright yellow on top. In most situations it should be noticeable from above. (Except above some crops; I once flew high over a yellow Air Tractor and he disappeared as he made spray runs over a field of Canola.)
I used mid green on the bottom, so that it would be stand out against a bright sky or cloud.
In case I prang it and end upside down there is also a wide yellow outline under the wing and the tail plane is white underneath.
As well as Kuntzelman strobes on wingtips, I also run bright Wig Wags, controlled by this great little unit: