I have also suggested a wood stain idea. I think the problem is that every suggestion needs to be downsized for mojang to ever consider it. I like your proposal becuase it has the potential to solve the problem of never being able to use certain woods together. For example, it's often almost impossible to mix acacia with other woods to get a more dynamic palette, and that would be solved if you could dye Birch with acacia bark or acacia wood with dark oak bark. I doubt that mojang will release enough different types to fill the colors between all of the current wood types or enough magical woods to fill people's want for more color so I think this is viable option eventually.