I just tried out your library, it does seem like it switches fairly seamlessly if you run the following command in the pytradfri api, changing the values for X and Y:
api(lights[5].light_control.set_xy_color(39000,39000))
Tried it with several different values which seem to independently dim the different coloured LED’s.
The negative side is I do get some timeouts if I hammer the command too often, so it might not be practical as it stands to do quick color transitions through many values.