Controlling a zigbee RGB puck light with MQTT

Setup, Configuration and Use Beginners
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I guess (without knowing your exact setup) you might need a transformation to get the MQTT Values into a color item of openHAB, f.e. like this JSONPATH Transformation and zigbee2mqtt - ColorLightbulb

And if you have that color item updated automatically you could forward those changes to your zigbee bulb. Maybe by “following” the channel or by using a rule