Currently it is possible via HTTP API to control colour & white channels independently, i.e. led.local/win&W=255.
I've been advised that HTTP method may not be around long term so opening up this feature request.
The use case in my scenario is to have effects, colour or ambient lighting on my led strip(SK6812) and to turn on/off the white channel independently with a PIR sensor when working in this area.
It may be possible with outside automation via home assistant by reading currently values, editing and sending back, but this is not as efficient as can currently be acheived via HTTP API using presets on board WLED.