I'd love to be able to fade colors and temperature like brightness level. Either holding a button to ramp or press a button to add a value to the current status. As far as I can tell there's no way to currently do this within Hubitat.
Some drivers support passing an optional rate key to the setColor() map, besides the standard hue, saturation, and value keys. This will let you specify, in milliseconds, the duration of the transition. This how the built-in Groups and Scenes app manages to do this if you specify a transition time for the scene. You can also do it from a rule (with a custom command, I think--I don't think the "standard" action supports rate) or a custom app. For example:
setColor([hue: 0, saturation: 100, level: 100, rate: 5000])
...will change to red (hue 0, saturation 100) at 100% over 5 seconds.
However, not all drivers support this, and this only handles color, not color temperature. And as mentioned above, many apps don't really provide a way to specify rate here either, nor is it apparently a "standard" key in this parameter (it's not documented, and I only know about it because I've looked at Hubitat's example drivers and the reason it was added to them).
So, your feature request certainly still stands. Personally, I wish there were even more options here, like the ability to specify a color temperature and a level all in one command, which for some devices would make sending these commands less chatty and possibly more reliable. But, in the meantime, you might not be 100% out of options. 
(I should note that you could also use a custom app or rule to fade/transition something over time via either built-in or manual actions, but that's not quite like having something built in to the driver.)
