From the code for the runImageJ module:
input_image_name = self.current_input_image_name.value
img = image_set.get_image(input_image_name,
must_be_grayscale = True)
So yeah, looks like the way pixels are passed back and forth, CP restricts you to only using grayscale images. You could tinker with the code to see what happens if you set that to False and try to use it on a color image, but my guess is it won't actually work. Sorry!
FWIW, work is actively ongoing for reintegrating the two pieces of software in future versions- if you want to find out if/request that it'll include compatibility for color images, you should check out the GitHub issue I just linked to.