Jaundice. Natural pigmented chemicals can turn the tongue colors ranging from yellow to black. This can cause the tongue to take on an overall yellow discoloration. Although this is the most common cause of a yellow tongue, occasionally a more serious health problem such as jaundice can cause the tongue to turn yellow. Jaundice can indicate an undiagnosed liver or blood problem so see a doctor.
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).