Jeff-
That could just be some minor variation that does not indicate disease. As long as you are providing adequate water, bright light will not result in overexposure: most bamboo prefer getting lots of light.

Bamboo have very few diseases. The main one you need to be concerned about is bamboo mites, which can cause unsightly mottling, too. Below is a link to a picture of what bamboo mites can do, but I don't think that's the issue here.

Here is a link that might be useful: Bamboo mites

I assume a spray every now and then with a solution of rubbing alcohol and water couldn't hurt to control any mites that might be there?

Jeff-
It probably won't hurt, but it wouldn't cure a bamboo mite problem. They are very tough and need a particular type of expensive miticide, like Avid, for control.

My black bamboo has bamboo mosaic virus - and as the result, the leaves are mottled. Obviously, I don't give starts to folks. The 20 year growth is more than I care to give up - so I just "live" with it.