CGB, the conspicuity trials were written up at the time, and remarkably, showed no improvement and actually some worsening. Counter-intuitive, I know. The authors thought that perhaps the dayglo stripes acted more like camouflage breaking up the outline than aiding visual acquisition.
But in any case, as BGA Laws and Rules has it as a “recommended practice”:
RP25. It is possible that gliders may be rendered more conspicuous in certain conditions by applying large, bright coloured patches. As coloured surfaces absorb more heat and this can appreciably weaken the resin used in the construction of Glass Fibre Reinforced Plastic and Carbon Fibre Reinforced Plastic gliders, it is strongly recommended that before applying colour in such cases, the guidance and advice of the glider manufacturer or the BGA Technical Committee is sought.
The only glider manuals I have seen mention it, limit paint to tips. It would be a brave inspector/BGA person to go against the manufacturer of an EASA glider, IMHO. Dunno about the RAF.
Chris N.