An excellent question from RansS9, and a thoughtful set of replies.
I too was flying yesterday, and we heard about the collision not long after it happened, not far to our south: it certainly provoked much thought and discussion, especially as the high cloud base and gin-clear air seemed pretty much ideal for aviation. It often seems to be the best days when the collisions happen, though: everyone gets their aircraft out (our hangar was almost empty yesterday, for the first time in months), and presumably the probability of a collision depends upon the square of the number of aircraft aloft. But one also wonders whether there's a 'first day of spring' effect, and not all pilots are as current as they might be. Perhaps the clear vis. also leads to a degree of complacency. (I stress that I'm musing in general, and not thinking of yesterday's events.)
It's also true that the Brize-Benson gap is pretty busy, as are all other bottlenecks: Keevil, Chilbolton, routes to the Isle of Wight... Maybe there's something to be said for *encouraging* GA to route through controlled airspace when it fancies, but the odd transit refusal and widespread disdain for unpolished R/T put most GA pilots off, so they never get to practice it either.
Aircraft colours: white shows up well from above, black from below (though most aircraft look black against cloud) ... but I'm not aware of any conclusive findings about the ones at your own level that'll get you. I've never spotted an aircraft from its strobes but, on the other hand, landing lights can be very obvious, though they must consume a lot of power.
Would Mode-S/TCAS/Flarm help? My guess is that if every aircraft had it (presuming low power versions available etc. - don't start that debate!) then you'd want the clever algorithms of Flarm to make much sense of the dense mixed traffic on a sunny summer Sunday. But Flarm has only got where it has because it's been unofficial: as soon as it became a defined and regulated system, even if targetted at GA specifically, that innovation would all but stop.
All rather tricky...
Windrusher