Hey, I'm pretty new to Java and modding, but I think you can't make a block with changing Textures, (I tried that and its changing the textures for EVREY block, not only the one you clicked (but I'm probably just doing sth. wrong ^^)) so, however, I think it's a lot smarter to just make diffrent Blocks for every colour, wool and the furnace do the same thing too, then just check in the onBlockActivated() method, what the block was clicked with (I'm not too sure how you do that) and set a new Block with the right colour to the old position...
hope that wasn't confusing and i could help