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I have seen the picture you have attached (attachment removed to protect patient identity). This is 100 % normal for the tongue to look like. The undersurface of the tongue looks in this color because of the artery and veins supplying the tongue is quite visible clearly. This is the same sort of thing seen in the inner surface of the elbow joints. And the string running through the tongue is the muscle attachment called frenulum which again is a normal feature. Do not panic for this. You have told you have never seen it in this color. But I think you have never probably seen this keenly. Moreover, it is the way we see things. You should be worrying only if your tongue turns red, purple or even matte white for that matter. The top surface of the tongue will still look whiter in most of the normal times because that is the part which carries the taste buds and oral bacteria all around. I mean you should not see white patches in your tongue. Good oral hygiene includes tongue cleanliness too. I advise you to use a plastic tongue cleaner or back of the toothbrush head to clean the tongue after every brushing.