Dental Health - 2 year old Lab

I have a beautiful Black lab, who just turned 2 on the 18th.

I was looking at his teeth today, and I noticed that his lower incisors show
some signs of wear. Specifically I can see two distinct colours on the top
surface of a couple of his lower teeth. The inner colour is grey and only
very small, outside that is white. His upper incisors don't show the same
pattern (they are all white, and 'sharper').
The rest of his teeth are pretty much all white (actuall there is a small
band of plaque along the gum line on his back upper teeth, but the gums look
heathy, at least to my untrained eye).

Is this normal for a dog his age?

He gets an occaisional (~weekly) soup bone to chew on, and mostly Kongs
otherwise. I do play an occaisional fetch with tennis balls, but not
generally in the water. Mostly they are found tennis balls, which we play
with at the park, and leave there when we go. I don't buy him tennis balls,
but I figure an occaisional play with a found one, maybe once every couple
of weeks shouldn't hurt (he generally doesn't shred them, just carries them
around for a while.).

Mostly in the water we use sticks, which he sometimes enjoys pulling apart.

I tried to get a picture, unfortunately I had a hard time getting his mouth
open with one hand and taking the picture with the other.

Thanks,

Dale