The tongue is a highly vascular fleshy organ which gives a place for bacteria to grow. These bacteria include gram-positive and gram-negative bacterial groups. To your surprise, not all the bacteria are bad.
A healthy tongue is pink in color with the papillae covering the surface of the tongue. If you are suffering from a painful tongue or altered tongue color, then it is the point of concern. Visit a doctor in such condition.
Most of the reasons for a yellow tongue are minor conditions that are treatable. Your tongue may become yellow because of problems like Jaundice, oral infections, geographic tongue, and black hairy tongue.