Not sure how to describe this. Trying to work on a Win7 style here. If you download the this archive,
http://7z.vc/VSBWin7.7z there are three files; 3 PNGs and a GIMP XCF. This problem is occurring on v 1.4
Two of the PNGs are very similar, in fact the left column on both is identical as I copied and pasted it trying to track down the problem. When when these two images are imported in to replace the Start menu Programs and Programs extend panels one of them imports significiantly grayer than the other. Trying to load both in else where, I randomly tried as a window frame, both import at the proper hue. I also tried switch the order in the start menu panels, i.e. I loaded the expanded graphic as the nonexpanded panel and vice-versa and again the same image appeared grayer.
So the second test would suggest the problem is tied to the image file, yet that the problem doesn't manifest when tried at another random location is baffling.
The included problem.png explains better than my words. The image in the bottom right screenshot is clearly greyer than the top right though both imported images are identical.