Here's one method.
Draw a rectangle covering your entire slide. Set the fill to be the color you want. Push the order of the rectangle to the back of the slide if you don't want it to cover the slide content.
Next, on the Formatting Palette, click on the Custom Animation button, Add Emphasis button (the yellow star button), and select Blink. For the animation settings, set them as follows:
Start: With Previous
Speed: Very Fast (or the speed you prefer)
Timing: Delay = 0 sec.
Repeat: Until Next Click
This should cause the shape (effectively, the slide) to "blink" repeatedly whenever the slide starts.
Not sure if this is quite what you want, but play around with it and see how it goes.
(BTW, the above steps are for PowerPoint 2008. I don't have version 2004 installed, so your steps may vary, but this is the general idea.)
Jeff
