It's a bit hard to figure out how to do something when FileMaker doesn't it make it possible to do it.
You can only select from a series of "color styles" on the styles tab in chart set up. If you can find a color style that works, you might be able to do this. If you can't, you can't.
It's a bummer that we can't choose the color ourselves. Not conditional formating. Maybe in the future. Thanks for your help.