Recently I wanted to have the bubble color change with the bubble size. Here's what I did...
Scatter plot Dimension: [Employee Name].
Scatter plot Measures: [Sales], [Years Employed], and [Quota] (all of these are being Summed).
In the color by expression field, my formula is:
Colormix1((Sum([Quota])-$(=min(aggr(Sum([Quota]),[Employee Name]))))/$(=(max(aggr(Sum([Quota]),[Employee Name]))-min(aggr(Sum([Quota]),[Employee Name]))))
The gist of the Colormix1() function is that you need to give it a value, then a start color, and an end color. Note: there is also a Colormix2() function which gives you the ability to set a midpoint color.
Hope this helps. See the first sheet of the attached qvf file.