This seems like a simple enough request, however I’m not experienced enough with Python to make the changes myself.
I’d like to request that the Phillips Hue component add an optional positive_int field to it’s service call to allow passing a transition time. The Hue API supports this for Group Set action (field is “transitiontime” and is uint16) and also for the Light Set action. The field takes a time in miliseconds for the transition to take place, and if the parameter is not included with the Set action, the default time is used (which is either the system default 4ms or whatever transition time was built into the scene being requested).
One simple example of this feature being used could be turning on lights slowly at sunset. I have an automation that turns on some lights 30 minutes before sunset, but on a cloudy day when it is darker out to begin with the lights do not keep up as well. Using a weather sensor (or a light meter sensor) and templating, I would be able to dynamically change the transition time, ensuring the proper amount of light for any given situation.
HASS currently appears to not include this parameter in the API call to the Hue Hub, which the hub will interpret as “Use transition time stored within requested scene, or default 4ms if not stored with scene”.