Change Spreadsheet cell colors?

In case you are indeed referring to Numbers app:

1. Highlight the cells
2. Press OPTION_COMMAND_I to show the inspector
3. Go to the 4th tab from right for Graphic settings
4. Choose fill type from drop down (solid, gradient, etc...)
5. Click on the rectangle beneath it (red I my case, white in your case)
6. It will open colors pallette - pick a color

You can also achieve the same by adding the color fill pallette icon by customizing your toolbar

Here is the second approach, if you can see the FILL button in the toolbar - red in my case, off to the right.

Select cells, click this buttona dn a small pallette will pup up.

If you want to remove a color, choose the white color which has a red diagonal stripe across.

Are you referring to the Numbers app?