Painting a glider black :

the UV absorbtion and the Temp of the gelcoat might alter the life of our precious machines !
If I remember the the USAF has a/c painted black : SR71, B2, F117, etc.. not to be seen....
The strobe light does not work on bright sunshine = 90% of the time when gliders fly , ( except possibly in the UK

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The low cost / low weight / low consumption SSR for gliders exists already : Becker has one with mode C encoder and antenna for 3000 euros. But it is not mandatory (YET)
No need for TCAS at 50.000 Euros for GA, ADS-B displays will do the trick for 3 or 4.000 euros when they will be available ,
Best trick at the moment : put orange reflective tapes around the nose, wings and the end fuselage, this does not affect drag, and cost very little. In many clubs in the Alps this is mandatory.
The BEST way however still remain to LOOK OUTSIDE all the time ...