Red line on gum on frontal tooth veneer. Had veneer replaced, and red line came back! What's going on? (photo)

There may be a violation of the tooth’s  personal space within the maxilla bone termed “Biological Width” ie.  You should consult with your dentist and a periodontist to evaluate and consider your options for a clinical crown lengthening procedure.

You may have an invasion of the biologic width which would cause the area to turn red. They grey appearance isusually due to a dark root color showing through the tissue. Has your tooth had a root canal? If the answer is your it could be the cause of the discoloration.

Three things come to mind:1) Encroachment of the Biological Width ie, the periodontal gum pocket needs to have 2-3mm distance from the facial bone, if not, it may stay inflammed.2) Your bite is traumatic and you may have parafunctional habits possibly while sleeping, very common.3) Rare, allergic reaction to dental materialsYou will have to work with a dentist and/or periodontist to improve.

Looks like inflammation. Have your dentist check for excess cement under the gums. The gums really hate dental cements.