Bacteria, fungus, and mold spores normally found in the air can cause rings in your toilet bowl. Wet surfaces provide an ideal environment for these organisms to live and reproduce quickly, growing together to form a ring. The color of the ring depends on the type of bacteria, fungus, or mold that settles on the wet surface.
You can easily remove the rings with common household cleaners and a toilet bowl brush. Keeping the toilet lid closed as much as possible will help minimize the number of spores that can enter the toilet and reduce the amount of light that the spores need for growth.