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.
Paul's Down-Home Page 
Reblogged this on Bringing All The Fail and commented:
This was literally driving me insane. If you’re using the new Outlook for Mac and are wondering why the hell the folder pane keeps changing colors at what seems like random, this clears it up. I’ve tested and confirmed this is correct; the background elements will bleed through a bit with their colors.
I was experiencing this “feature” thinking there might be an issue with my GPU, but luckily a Google search led me to this page. It turns out that this is actually a feature of Mac OS X and not Outlook. Fortunately, this option can be turned off by going to System Preferences> Accessibility>Display>Check the box to reduce transparency.
That’s awesome, thanks for sharing this and commenting directly on my reblog note so that I got a notification. I appreciate the heads up!
Thank you! This solved my problem.
This is a Mac OS X feature. You can turn this option off by checking the “Reduce transparency” box found here:
System Preferences>Accessibility>Display
Phwew. I had a funky webpage behind Outlook and it was a repeating gradient that completely looked like a busted LCD screen. Was making me a little nervous with my new MBP. Quickly experimented with a few different full screen colors to verify. Not sure if I like the feature, but good to know my LCD isn’t trashed.
This was really driving me insane. I thought that Outlook was trying to tell me something through the color scheme (reds, blues), but no, it was just picking up the background image. Thank you!
Thanks – this was also driving me crazy.