A fixture overwrites value of previously created if placed to the same channels.
I think this is a feature, and here is an idea how you could use it.
Let's say you want to control moving heads, which have rgb color mixing on their 3-5 dmx channels.
I made this patch where you can map controls of your moving head. viewtopic.php?f=14&t=12513
Map the moving heads to the correct grayscale, with layer input.
Place those mapped lights in the correct dmx channels, then create a single fixture which has the same color space as your moving head.
Map the color fixture to your composition to the place you want to get the color info from.
Now place this color fixture to your dmx output to the 3-5 channels.
Automate the remaining channel values in the patch clip.
Now you have a moving head entirely controlled from resolume with color from video!