I mentioned in my review of the new Outlook for Mac client that the background of the folder list seemed to randomly change colors:
It may also be a feature that there is a color gradient fill in the folder list. At first I thought the color was the same as the color of the category of my current calendar appointment, but after changing all the category colors, waiting for sync, and quitting and relaunching Outlook, the color didn’t change, so I’m not sure what Microsoft had in mind here, and there doesn’t seem to be a way to turn it off.
Thanks to the most excellent Bill Smith, long-time Mac Office MVP, now I know the answer:
You’re seeing translucency in the navigation pane. So long as you have a window or other white object behind Outlook you’ll see a whitish background, but arrange Outlook over your Desktop picture and you’ll see those colors peeking through it. Choose Outlook menu > Hide Others to quickly show Outlook over your Desktop.
Sure enough, that explains it. I use SatelliteEyes to update my desktop background, and as I move around (and thus get new satellite maps), or as change the Z order of other open windows, voilà color changes. I normally don’t mind window translucency, but I don’t care for the combination of OS X Yosemite and this effect. Looks like I’m stuck with it, though.