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The normal color of tongue is reddish pink and the normal texture is seen to be velvety and smooth. Hence, when there are tongue sores or any kind of infection of the tongue, then the color and appearance of the tongue changes.
A green tongue could be a indicator of a underlying bacterial or fungal infection or other diseases. Some important causes of a greenish discoloration - include - hairy tongue, oral candidiasis, streptococcal pharyngitis, etc.
You may consult with your doctor for a physical examination and proper treatment.