Describe the bug
Attempting to pass a cmyk color channel to visualize.pixel_scatter_plot and threshold.dual_channels results in a fatal error (channel not valid). This was encountered when following the multi-object tutorial.

Expected behavior
From following the multi-object tutorial, I would expect any of the color channels shown by visualize.colorspaces to be valid inputs for plotting and thresholding. The error message given by both functions does not mention cmyk channels as valid inputs.

Additional context
Support for cmyk channels can be added by updating channel_dict and the fatal error message.