Although I don't think more colored blocks should be something to spend more focus and time on, more blocks really only increase creativity... I also really want dark grey planks so I like and support this idea.
On one hand, this would seem to be impossible in the current game unless you want to just add colored brick and colored wood blocks, which would seem to defeat the point of adding paint items. Why not just craft the bricks with dye?
On the other hand, being able to paint individual block faces would be immensely useful. I would definitely consider it useful if I can color the inside of a house while the outside still looks like the material the house is built out of. And no, building 2-meter thick walls is not an option since they make windows and doors look strange and take up far too much space.
I support the concept, I just don't think it's possible without Mojang inventing some new weird new non-block non-entity objects to the game.
I get offtopic a lot, be warned. I also often mention a Minecraft clone called Minetest. The clone is nowhere near playable with it's default game in my opinion, but it's at least the start of a good open-source voxel rendering engine, and maybe one day it might be decently playable.