We already have custom movable and water colors. And I think everybody saw Eric Gurt's twit about custom lamp colors. But how about other things?
1) Lasers. It will be cool.
2) Grass. It green even under snow.
3) Barrels. Maybe? Or its another story...
4) Explosions. Green, like BNG, or violet?
5) Teleports. They have glow part. Why to not recolor it?
6) Water. Yes, we have this. But we can't recolour ten certain water regions in different colors.
7) Backgrounds. It would be good to have one changeable background instead of just four different colors.
8) Shadows. It's for other things: stones, walls, backgrounds with not-continuous texture. For example you want red stone instead of grey. Or wall. Or tile in base.
Concerning the last someone will say: "But you can simply make fake water over the wall or background and change its color". That's right, but water's color cover every object in water area. Also u can’t make more than one color (or two?) using this trick.
Hmmm...seems wrote everything I wanted.