If you a re experiencing pain, you should address the problem now. It will never get any easier or any less expensive than it is right now.
If it's not causing you problems, and the dentist said it looks ok on examination and on x-ray, then it's ok to leave it alone. The canines are typically a little darker than the teeth next to them.
Ask your dentist what that sign means. To my knowledge, the bonding materials used pose no threat to your health. Unless they were talking about the metal amalgam material.
You could have a sensitivity to cinnamon or mint, inform your dentist of the occurrences to make sure it is nothing else.
IF you're a candidate, you will most certainly have better results with professional whitening. You risk extreme sensitivity with professional whitening, other than that, it's safe.
Without seeing you for an exam, it sounds like what you said, an irritation. If it doesn't get better or gets worse you should see a dentist or an oral surgeon.