Why are the wingtips of sailplanes painted red?
White is indeed chosen for thermal reasons. Gliders use epoxy resins which cure at room temperature and are then tempered at 60°C. This tempering shifts the glass transition temperature to something close to the tempering temperature, so the structure must not be heated under stress to more than this temperature. To limit heating by solar radiation, black surfaces should be avoided altogether and mostly red, blue or grey is used for markings.

The red wingtips were popular in the 1970s and were meant to improve visual identification when flying in hazy weather and close to clouds. They can be mostly found on gliders of this era. Since structural loads are lowest at the extremities, wing- and fuselage tips can be painted in high-visibility colors.



Schempp-Hirth Cirrus with red wingtips but no markings (picture source). This makes it obvious that this is a British glider.