OK. but lightness does work via
convert rose: -colorspace HSL -channel Blue -separate rose_lightness.png
convert rose: -fx "lightness" rose_fx_lightness.png
compare -metric rmse rose_fx_lightness.png rose_lightness.png null:
0 (0)
But none of my luminance comparisons work! (see ammended notes above). Should there be a one-to-one comparison with at least one pair?
Please correct my conversion syntax if they are wrong. I am not sure what to do with the rec..luma colorspaces or the luminance channel!