Sometimes color has to do more with concentration than impurity. Case in point: take some white/clear crystals and melt them down gently to form a nice little rock. It will probably turn off-white if not yellow. Start with yellow and you'd probably end up with something orangish that, while probably harsh, will blow your top off.
And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove that what is good, and acceptable, and perfect, will of God. Romans 12:2