IMHO the only people qualified to express an opinion are active glider and power pilots: rich_hodgetts et al. Anyone who has experience in only one 'discipline' lacks necessary perspective.
Fibreglass gliders must be white, period, for anti-UV reasons (ToryBoy, I presume that you have a D.Phil. in FRP engineering?
). But I like david viewing's idea of some sort of reflector panels ... that might help, and probably wouldn't cost much in terms of dollars or performance.