Red in human tonguess is because of hemoglobin, but as pointed out in answer, different colors in other animals come from pigments.
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Andrew Grimm
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Are tongues red because they contain blood?

Many animals have red tongues, though not all of them - blue-tongued skinks and giraffes come to mind. Are they red because of the blood (and therefore haemoglobin) in them?

It sounds plausible, but it also sounds like a post-hoc explanation.

I'd be especially curious about any other species having blue blood, like horseshoe crabs, but still having red tongues, unlike horseshoe crabs, which apparently don't even have tongues.