Two personal experiences:

In one of my clubs, an ASK-13 was painted completely in bright orange. The only situation where this was really visible was in bright sunlight when flying slightly higher. In all other situations it just looked gray/black, just like any other sailplane.

During one alpine flight, with a green/brown backdrop (no snow) and with the sun in the back, I was surprised at seeing a series of white dots coming in my direction. It was a glider with a series of hi-vis stripes, approximately 50 cm wide, taped chordwise every meter or so on the leading edge. The white was decidedly more visible than the stripes...

With a snowy backdrop, bright orange markings are a welcome plus, but I prefer large markings at the wingtips rather than numerous small strips all over the sailplane.

I used to fly in a Janus with two classic white strobe lights, one under the nose in front of the nosewheel, the other on the back just behind the canopy. No improvement in visibility in full sunlight, but very visible when flying in the shadow of the clouds (cloudstreet, convergence). Not sure the new "LED" strobes would be as efficient, but certainly worth a try.