So, I tested the converted cable, except for the picture quality being really bad, the colour was fine no matter what resolution I tried.
I then decided to convert a composite+stereo to SCART adapter into a RGB RCA to SCART adapter and tried on both a Xbox and a Xbox 360. Everything looked great. So unless there’s a difference between Sony’s and Microsoft’s YPbPr signal, it has to be the cable.
Can’t quite grasp how a straight copper cable can work with some resolutions and encodings and not others. Is there a good way to measure the blue channel to get a definitive reading?