It works multiple ways, I think RGB is the easiest:
zigbee2mqtt/item/set/color with the value in RGB hex (so #FF0000 will be full red). I’m currently trying to figure out how to make it work with the colorpicker because it works from mqtt but the colorpicker wants to send these hue & saturation values that I don’t fully understant.
It also works with JSON providing the values in this format
{r:“255”,g:“0”,b:“0”}
