Pete, when the paint flakes off and all you have is silver-grey metal on the paddle and around the kill, it doesn't make any difference what colour the target is.
White or yellow faceplates are great for fast target acquisition especially in dark areas.
I was shooting some black targets with white kills in the woods on Saturday and the paint was flaking off very quickly and nearly every shooter said the 15mm was nearly impossible to make out the kill at closer ranges. Good eyes and good scope help a lot.
I shot with my springer and EB first and it was very hard to see the kill. Tried later with my the Steyr and Bushnell and it was slightly easier to make out.
I do remember one 18 or so yarder 15mm at Wendover in a dark area where it was very difficult to make out the kill when it had been shot up.
I did try preparing faceplates very carefully last winter by blowtorching the old paint, wirebrushing on the drill back to bare metal then priming with good quality Halfords grey and then painting with Halfords satin. The paint didn't flake off during comps and you could see individual pellet strikes more clearly but Jeez it was a &*^% of a lot of work!
For important comps, it could be worth doing, I took the targets home and did a few each day, so it wasn't too bad.