In spite of a number of composite airplanes showing up around the country with various color schemes, RAF does not approve any color but white. Trim colors must be limited to the vertical surfaces. Do not paint a dark stripe on the top of the wing. Since the introduction of the European glass sailplanes in 1961/62, there have been no cases of the composite sailplanes having any damage from heat, and all of then are white. They cannot be certificated if painted any other color.
There can be no doubt that a dark color will reduce the useful life of your airplane in the lonq term, and could lead to even more serious consequences should the structure get too hot. White guarantees that your airplane will never go above about 10 degrees ambient air temperature.