Hi Neil;
Via the Report Painter itself - No. It might be possible in IM2022 as PB & thus IM will get a Graphing feature boost per the roadmap.
You can do this in PowerBuilder because you have 100% control over any graph's colour settings program (PowerScript language) wise. However, a workaround could be to create a PB global function that can set the graph's colour(s). GF's in PB can be then copied to an InfoMaker's library (aka a PBL) and then used internally by IM. I have not tried this personally but it should work as I have used PB GF's before in IM to perform report (aka DataWindow) property updates in real time.
Regards ... Chris