grayish spots on front teeth?

I would get the dentist to do an x-ray. After I got my braces off as a teen, one of by back was grayish looking, but I kept being told that it was just a stain. Then, 10 years later, a new dentist looked at my x-rays from the previous dentist and immediately suspected that it was decay. He drilled it, and sure enough, half the tooth was rotted away. He ended up putting a huge filling on it and said that it will likely need a root canal and/or crown later on, though so far I haven't had any problems with it. This was a back tooth, so I'm not sure if the same would go for a front tooth, but be sure to mention that the spots have been growing.