It depends a lot on whether you are looking at the other a/c from above or below. From below, solid black has to be the best colour to stand out against the sky, white the worst. Dayglo patches on white undersides would help.
From above, solid white is probably one of the better colours, with black worse and a disruptive pattern, even dayglo probably not helping. I have found that spotting "high viz black" RAF traffic from above can be very difficult, especially against dark woodland backgrounds etc.
I believe that World War II Coastal Command aircraft significantly improved their U boat kills when they were changed from night bomber (black) undersides to white. U boat crews generally look at aircraft from below.
The argument about composite aircraft having to be all-white to minimise heating may be valid for the upper surfaces,but has to be less so for the undersides.
However most composite a/c manufacturers are very careful to specify that only small areas of relatively unstressed wingtips, nose cones etc can be anything but white. No dispensations for undersurfaces.
I don't know how the ATC got away with the dayglo on the Vigilants etc. But unless the composite manufacturers change their specs, most plastic aircraft will have to stay all-white.