Every thought of yours has a color in it. I think the modern science even proves it by taking pictures of it. Your thoughts can be pictured. An angry thought is reddish in color, a sad gloomy color is cloudy, gloomy. A bright, nice thought is shiny, golden.
Every thought creates a color, and that color is transformed to the body also.
When your face glows with nice - your mind seems to be happy. An unhappy mind creates unhappy thoughts and reflects in the face - you see an unhappy face. So that means, if you want to keep the mind calm and clean, at least lets have clean thoughts first that will not disturb the mind. Anything that disturbs your mind is unclean. That means we have a lot to work on.