I own a ZX81 but I think I never heard of this color interface. It’s even supported by some emulators like ZEsarUX. Some games/programs are newly available in color. There’s also a documentation for own software. But maybe b&w is the real retro.
I found this by coincidence searching for a specific DIN 41612 connector. Surprisingly someone has this industrial connector on ZX81 cards (XBUS)
https://forum.tlienhard.com/phpBB3/viewtopic.php?t=3246&start=25