In my experience stress bars in 'toos are generally lighter or darker lines appearing through the feathers. In some cases in U2s I've seen them actually as thin, almost see through lines. Generally they molt out, or grow out w/ the feathers and are not usually a cause for concern.
I liked this article for giving some clarification of stress bars as opposed to rough play, feather growth complications, etc.
There is another article HERE by Eb Craven @ the Gabriel Foundation.
Occasional stress bars are not uncommon, but if you see them regularly, or a LOT of them, it should be cause for concern and you should make a trip to the vet. Stress bars can occur for a wide # of reasons, like poor diet, other health problems (liver damage, breathing problems, etc.), not just plain stress.
Other health concerns can also change the colors of feathers growing in ... example ... a Blue & Gold macaw w/ a liver problem may streak green and orange into the feather growth.