Someone on ARC was asking about the color change in yellow watchman gobies. I can only say for sure what I learn from my personal experiences. Wilma was the first fish I ever bought. That was 5 years ago. She's always been a nice yellow. Here's a shot from March 2011 when I introduced Fred to the display tank. Wilma is the larger of the two, on the right and slightly darker. You can see in the post "Wilma" that she's still pretty much the same color. This month I've noticed a color change. Here's a look at Fred and Wilma today.
Now I've seen some washed out looking gobies at stores and always attributed it to stress and possible age. This isn't that dramatic a change, but a change nonetheless. So is it age, fertility or something else? The offspring are that typical bright yellow.