Can't change color_mode

Hi,
I got a Zigbee dimmer working/found in Home Assistant. It’s just a dimmer and doesn’t support color from the hardware. However, Home Assistant claims it does, as it allows me to set a color in the Dashboard. A look in the core.entity_registry file reveals the following:

"capabilities": {
     "supported_color_modes": [
           "xy"
      ]
},

Manully changing it to brightness doesn’t change a thing, restarting Home Assistant just changes it back.

Why can’t I change the color mode?