Old dog tongue has turned half brown

My dog has no teeth and his tongue is turned half brown, vet told that its because his tongue always hangs out.

Hello - its certainly true that dog tongues can hang out and dry out and that can result in a change in appearance to the protruding area; dog tongues are supposed to remain moist all the time.  Your vet sounds quite clear on what they think has caused the changes in this case; I wonder whether this is down to the teeth, the nerves supplying the tongue, or both.  It can certainly be difficult condition to treat.  Wishing you all the best of luck.

I think that good questions for your vet at this time might be, 1) what has caused this in this case?  2) can it be treated?  3) is my dog uncomfortable and should we be doing anything to alleviate this discomfort at this time.