Looks like the old Windows folder already got deleted but deleted the rest anyway.
I did also try deleting the config file for deathloop which did make it boot in colour, but it was in 1080p. Setting to 4K again still retained colour and worked, then I exited to Windows and found the desktop was black and white and booting deathloop again was black and white again.
Not sure whether to blame the DL update, W11 or the monitor at this point. I have heard of things going black and white for the monitor before but this is the first time I’ve seen it.
UPDATE: OK just tried Far Cry 6 instead and same thing. So doesn’t look like a Deathloop only issue. Narrows down to W11, monitor issue or maybe even latest GPU driver, but not seeing widespread reports about it.
UPDATE 2: Believe I’ve managed to fix it using DDU to wipe 3090 drivers and installing the last version 472.12
Now it seems to boot in colour on Deathloop and Far Cry 6 (though Deathloop still has the splash screen issue not appearing but I can deal with that ha) so either latest Nvidia deiver causes it (which others could confirm?) or my drivers in general seemingly caused it.
UPDATE 3: Yup latest Nvidia driver to blame. Installed latest version again and black and white screen appeared again. So Nvidia/Monitor issue I guess?