That is correct. Colour relies on OIIO for image reading. I have had issues myself making that dependency work, so am not the best person to help.
But the plotting functions of Colour simply plot from a NumPy array of colour triples, so any way of reading an image into that will work. For 8-bit integer image formats you can simply use PIL.
EDIT: RGB_chromaticity_coordinates_chromaticity_diagram_plot_CIE1931() expects linear input, so remember to linearise the values in your NumPy array if necessary before plotting.