Causes of Grey Lines on Veneers 1 Year Post Op?

When bonding porcelain it is critical to control blood and saliva. If moisture is not controlled a grey line can show within months. It is likely not decay and only a cosmetic issue, but would need complete replacement to change.   If there was a shelf for bacteria to grow, the problem would be gums related and not a grey line issue.

I'm sorry you're having issue with them in such a short time.  Unfortunately, it could be due to a couple of things 1) Stain  at the margin 2) Decay because of a bond failure 3) Could be metal.  

I have seen many instances where people have come in to my office because they didn't like the appearance of their "Veneers".  When in actuality they were crowns .  And worse they were PFM crowns (Porcelain Fused to metal).  Probably not PFM's but without doing an examination, I'm just throwing it out there. 

Your dentist should look at the margins with high magnification to see if there is excess cement left on the margins.  Dark lines are either due to extra cement or due to veneers not tightly seated enough and leaving a small gap betweeen the veneer and tooth which traps food, bacteria and eventually becomes stained or decayed.  If the problem is excess cement, it's easily fixed, but if there are gaps, that may be difficult to correct without replacing at least some of the veneers.

Without knowing the history of your case it is hard to be 100% sure. However, I would agree that it is probably a bonding issue most of the time. Where you gums bleeding when the temporaries were take off?? How long where you wearing the temporaries?? It is possible that some blood or saliva contaminated the bonding procedure, and that the gray lines are showing that. Have any of the teeth had root canals?? Sometimes after a root canal the root can turn dark and give the tooth a gray appearance. If your veneers are over contoured at the gum line and rough, it may be possible to have the dentist recontour them, and see if that helps?? Do you smoke?? It could be possible the gray is staining of the resin cement used  due to smoking. I would recommend recontouring them at  the gum line first. However, be prepared if you do not get the results that you like, you may need to have them redone.

If the cause is  only the "bulkiness" of the margins, resulting in bacterial build-up, then it would be worth trying to polish the margins "razor-thin", and see if that helps. If the cause is "poor bonding technique", however, then you have no choice but to remove the veneers and begin again. You really have nothing to lose (except time and a few dollars) trying to "thin - out) the margins first, unless it's obviously a poor bonding.