The menus and dock in OS X change colour to give you an idea of what is under them. I’m sure some people find this attractive but it annoys me because it can make things less clear. Thankfully there is a simple way to turn it off.
Here is a picture of the dock in OS X as it normally appears. You can see that my dock has a pink colour because underneath it there is a background that has reddish tinge to it.
This is what the dock looks like after you turn ‘transparency’ off. It goes back to the old grey colour, which is a little bland, but personally I prefer it!
To go back to the original grey background, Open ‘System Preferences’ in the Apple menu then click on the accessibility:
Now click on ‘reduce transparency’:
That’s it! Now your dock, your menubar, and the tops of your windows will be grey.