Currently mqtt light template doesn’t support color mode.
It makes impossible to create light entity for bulbs like Shelly RGBW using mqtt.
For this particular bulb no other mqtt schemas work (neither default nor json) due to their limitations. So the only way to handle such a bulb is by creating two light entities. While it works in general, it’s the reason of problems in specific cases.
The best option would be to make all mqtt light schemas uniform in regards of supported features.