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Sounds awfully PowerPointish... Keynotes are designed to be sexy, not have a solid color text box. You could always insert a shape (square) behind the text box if you really wanted to.
 
Here's how I did it.

iWork '09

On the upper right is i button (inspector)

Clicked that on got a box

Two buttons Transition and Appearance

Appearance
Body
Background
Color fill
then I picked a color

Yes it is Powerpointish but just want basic information to be more like a billboard rather than a fireworks display.

Also I am using the same basic header information and copying the basic slide to build on was super easy. Now if I can figure out how to save it so the individual slides will usable in iMovie I will be golden!
 
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