Oh, wow, it's fn + L + Light Effect Key (long press for 3s). Thanks for the tip!

A full list of key combinations for the K3 (which I have) is at https://www.keychron.uk/blogs/news/k3-key-combinations - alter the url to suit your model, but most of them appear to be the same.

Same on my K8. It’s holding down something and the light buttons.

It’s a godsend, all I want is dim-ish blue lights, but I’d keep coming back to it doing rainbow patterns and flashes that I’m sure some people love but I find really distracting.