This doesn't work for the scatterplot (X,Y).
I have 3 data series and don't want both blue and purple (some of my audience is color blind). I'd prefer red to replace one of the other two. However, nothing I've been able to do will successfully change the color of the data points. With Keynote '08 I used to be able to copy style and past style from the series with the appropriate color, onto the series I desired to have that color and then delete the extra series. However, Keynote '09 insists that the first series is yellow, the second blue, the third purple, and the 4th red. I have to leave an extra & empty data series in my legend if I want to have red.
I've tried dragging the color onto the data editor, the data point, the legend, and anything else with no luck.