This section takes you through each step for changing the background colour in OSX 10.10 Yosemite with picture of the screens to help you.
1. Click the Apple icon , in the top left corner of the screen, then select System Preferences.
2. In the System Preferences window, click the Desktop & Screen Saver icon.
3. In the left-hand column, click the arrow next to Apple and select Solid Colors.
4. In the right hand panel, select a colour. The background should change colour immediately.
5. If you're happy with your selection, click on the red Close button to close the settings window.
Note: If this does not work it could be because someone else manages your computer settings (an IT department or administrator). If this is the case, you will need to contact them to access these settings or for further help.