See solution in other versions of Excel:
Question: How do I change the background color of a cell in Microsoft Excel 2010?
Answer: Select the cell that you wish to change the color of.
Right-click and then select "Format Cells" from the popup menu.
When the Format Cells window appears, select the Fill tab. Next choose the color that you wish to use as the background color or you can choose "No color" to return the cell back to its default transparency.
Now when you return to your spreadsheet, the cells that you've selected should have a background color, as follows:
(scroll to see more)