Haven’t looked closer into the problem yet, but my initial guess would be Blender’s vertex painting tools’ colorspace hardcoded problem. The painting tools seems to hardcode BT.709 primaries and when you convert the BT.709 red to P3, it’s not going to be (1, 0, 0).
Setting working space to P3 will expose more Blender’s hardcoded color management problems. Be ready. After we merge AgX to master, the Blender devs will be forced to fix those problems. But first folks need to actively make those bug reports to actually show them there is a problem.