That means, clean up the mind and make it free from any kind of waves in the mind. A waveless, calm clean mind is Yoga. To keep the mind clean and calm. Anything that you do to calm the mind, to clean the mind could be called Yoga. So when the mind is calm and clean, you see yourself clean.
Real spirit is reflected in that.
But normally, every thought colors the mind. You should know that. 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.