I noticed in Yosemite that Mail looked like it had a pale pink/sand color behind the mailbox source column and around message boxes in the conversation view pane. I thought that was odd and tried to change the color somewhere but there wasn't a preference for it. Mail was set to full screen. Then I changed mail out of full screen to windowed mode and moved it to a desktop with the old space background from pre Mavericks. Then I went full screen again. Now, wherever the pink/sand color was was now a bluish tint. Apparently full screen apps retain a level of translucency, borrowing their background color from the desktop that was behind them prior to going full screen.