That's because there is no such option.  You can either use a label instead
or use a bitmap to provide the color you want.

Stephan Lebans websit has a utility that provides for colored buttons.

There is nothing built-in to Access that will allow you to change the color of command buttons.
Here are several options for you:

1. Use a label instead. Labels have click events and can be made to look like a command button with
some work.

2. Sensei Stephen Lebans has an example database that shows how to set a command button back color
and more: http://www.lebans.com/cmdbutton.htm

3. There's a nice "Color Command Buttons" download at this site:
http://www.candace-tripp.com/access_downloads.htm

4. Some simple colored button examples that can be copied into your application are available at:
http://www.peterssoftware.com/clrbtn.htm

5. A shareware version of Button Painter for MS Access is available at this web site:
http://www.peterssoftware.com/bp.htm

6. MVP Arvin Meyer has some "hover" style buttons here:
http://www.datastrat.com/Download/MouseMove.zip