The yellow coloration is caused by tanins from the wood. What it does is create a lower Ph and a black water effect. Though it may not look very pleasing to most hobbyists, dark water fish like discus and tetras love it. You can soak it and boil it, but chances are you won't be able to fully get rid of that yellow color. As far as carbon goes, I've tried to steer clear of it. It takes out any medicine you may have to give your fish, can be toxic if not changed properly, and some beleive it is linked to some infections in fish.