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

I had a veneer placed on a front incisor in 2010. A month after it was placed, my gumline turned red/gray/purple. I hated it but was too afraid to get it fixed. Three weeks ago, I finally got up the courage. A new dentist said that my original dentist hadn't prepared the tooth properly. He took many photos and the lab technician seemed great and he put on a new veneer. At first it looked wonderful, but only a few days later the gum turned purple/gray/red again! What's happening? What do I do?