Changing overall background color of slides in keynote?

[anotherjeff]

I can't figure out how to change the overall background color of slides in keynote.
I checked the instructions in help and couldn't get them to work.
I open up the master slide, I'm using "Photo - Big"
I open up the slide inspector, choose appearance, background, color fill, and a color.
Only the gray box on the slide where the picture would go is now colored.
I want the whole slide to have color.

Do I have to make all new master slides in the background color I want?

Thanks for the help.