Yes, that worked really well, and it is persistent across reboots!
I was suspecting that there was something wrong with colour depth, as every monitor (internal and iMac) was reported to use a 30-Bit (ARG210210210) Framebuffer depth under System Report-> Hardware -> Graphics/Displays instead of 24-Bit (ARG888888).
Really simple workaround, as it does not require a reboot to apply, perhaps a fine-tune in EDID (if possible) will make it more "elegant", but that's something for later