OS X Yosemite lets parts of windows take on the general color of the background behind it. It's a translucent effect because details cannot be readily made out. For those who don't like it, here's how to turn it off.
In Yosemite, part of an app can, if so enabled, take on the color of the background via translucency. One example is the Safari search bar. Another is the sidebar in the Finder. Notice how, in the screen shot below, the Finder sidebar takes on the hue of the background: beige (at the top) and blue (at the bottom). This is the default.
You can greatly reduce this effect if it bothers you. Here's what the same shot looks like when translucency is off.
Managing the Translucency
This effect is controlled in System Preferences > Accessibility with a checkbox. The effect takes place immediately. (Why Apple used the term transparency here when distinct images are not visible in the background is arguable.)
System Preferences > Accessibility.
I like this feature. In previous times, the GUI of our OS was blunt, skeuomorphic and saturated in its color scheme. This color inheritance of the background is a pleasing, artistic touch that brings warmth and depth to the technical, almost clinical look of the Mac's GUI. Even so, Apple respects that some won't want it, and that it is managed in the Accessibility pane of System Preferences is a proper acknowledgement of some people's visual needs.