According to the flux LED component documentation…
White and RGB channels can be adjusted independently using a slider and color picker respectively.
I have setup my controller and see the brightness slider and color wheel and both do work, but the behavior is not quite as the documentation indicates. The slider does control the intensity of the W channel, but setting it to the lowest value also turns off the RGB channels as well. No way to turn off W channel independently of the colors.
Also, I would expect a control for adjusting the intensity of the RGB channels as well. Is there some reason the component has this behavior?
After posting this last year, I made a PR to adjust the behavior. My issue was that the W channel was not separate as it should be for an RGBW bulb. It sounds like you are looking for the behavior where W is controlled via RGB controls. If you have an RGBW bulb, then this is no longer possible via the HA interface AFAIK since it creates a dedicated control for the W channel and a separate brightness control for the RGB channels.
You may have some luck setting up a dedicated brightness slider and creating an automation that controls both brightness and W level simultaneously, but I haven’t tried anything like that.
That sounds like quite a tricky automation to write, I might try it, but I am not sure if it would solve the issue.
My issue basically is, that I cannot use both channels at the same time. Changing one, will turn off the other. In the Magic app, I can use both channels at the same time and control them separately, in HA I can only use one channel. I tried to create two lights, one for each channel, but the device has one IP, so that doesn’t work either.