Originally, radomes were black because it was a special radome paint that needed less preparation and primer and had less attenuation effect on the ground mapping radar returns. Later developments in paint technology included coloured pigments.
That's something I've heard before and sounds plausible so perhaps could be true.
Sadly though, it seems there just isn't one conclusive answer for the reasons behind the black radomes so I'll probably never know for sure.
With regards to paint schemes, I think that 558's gloss finish, although not accurate has stood the test of time pretty well.
Apart from the spruce up on the roundels and tail markings the rest of the paintwork is pretty much original and still looks good.
Same can't be said for the old wraparound matt scheme on XL426 however, which looks pretty awful up close these days.