I'd love to have more options for transparency in color settings. For example: Have labels use alpha channel so we can 'adjust' a font or background color with a label. It can be difficult to find a color setting for a label that works with all other label-colors, transparency can help there.
I think it would still be hard to pick a color and alpha value that works on lots of different colors while keeping things clear, if the colors were already varied enough that it wasn't possible without translucency.
In that kind of situation, I would make the label define both a foreground and background color, so you can know it is always readable/clear.
Leo, yes, picking colors is always difficult. Still, adjusting the foreground or background slightly (or sumsuch) would help me so much with all the different tags (and thus: colors) that I throw into my filenames 
Anyhow, feel feel to add this, if you want.