Modern (plastic) gliders are painted white (I suppose for thermal reasons), except for some shapes on the fuselage (generally blue or red, but it seems the color does not matter). And the aircraft registration is dark, I suppose for maximizing the contrast.
Now, for the wingtips I noticed that there are two possibilities:
white (nothing special)
My question is: Why red? Is there something special that is preventing the wingtips from being painted in another color?
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.
Personally speaking I like red wingtips because it makes seeing the wingtips a lot easier.
During a winch launch it is critical to keep the wings level which is easy in mid flight as you have a lot of time to look about and check the instruments, on the ground run of a winch launch (where fatalities do occur) the instruments are useless due to the bumping and you have no time to check anyway. Having the red wingtips means that you can see them in peripheral vision with a minor turn of the head.
Once off the ground in the launch, again it is important to keep level, there is no horizon (actually there is but it is underneath the aircraft and useless) and the horizon indicator may still be wobbling and takes time to check. Peripheral vision and red wing tips win through again.
Most of my club gliders have red wingtips but some have fluorescent colours.
Additionally reflective red or orange markings are required in Sweden when flying in winter conditions in the mountain areas. Stockholm segelflyg tradionally goes up North to Ottsjö, a bit outside Åre, for flying. Starting and landing on the lake ice. They (sadly I never made it there) fly in the upwinds created above the mountains when the wind comes from the right direction. Oxygen is used and I believe a few has reached 10.000 meters. It is supposed to be spectacular flying in wind rotors. Cold as well.