What can cause a yellow tongue?

Tongue discoloration: On the surface of the tongue are multiple papillae, which come in different sizes and shapes. These can easily be discolored by things we eat, drink and even inhale. The most common things that cause a yellowish discoloration of the tongue include tobacco (both smoking and chewing), coffee, tea and even brown sodas and other dark beverages.

Maybe thrush(fungus): The tongue is known to have many variations with pigmenting, size, appearance, & not always due to disease. Causes of tongue that looks patchy include: lichen planus (responds to steroids), white sponge nevus (familial, benign, permanent), smoker's leukoplakia (2% risk of cancer, needs biopsy), hairy leukoplakia (due to epsteinbarr virus), warts (due to papillomavirus), "thrush" (rx antifungals).