Separate the luminance from the chrominance in an image

img = Import["https://i.stack.imgur.com/2L3Hc.png"]

enter image description here

You can also use ImageMultiply or ImageApply as follows:

ImageMultiply[ColorConvert[img, "CMYK"], {1, 1, 1, 0}]

enter image description here

ImageApply[{1, 1, 1, 0} # &, ColorConvert[img, "CMYK"]]

enter image description here

% == %%

True

ImageMultiply[ColorConvert[img, "CMYK"], 1 - {1, 1, 1, 0}]

enter image description here

ImageApply[(1 - {1, 1, 1, 0}) # &, ColorConvert[img, "CMYK"]]

enter image description here

% == %%

True

Here's something that uses the "LAB" colorspace:

butterfly = ColorConvert[Import["https://i.stack.imgur.com/2L3Hc.png"], "LAB";

To kill the color channels:

ImageMultiply[butterfly, {1, 0, 0}]

enter image description here

Or to put everything on max "lightness" (an approximate luminance):

ColorCombine[
 Prepend[
  ColorSeparate[butterfly][[2 ;;]],
  ConstantImage[1, ImageDimensions[butterfly]]
  ],
 "LAB"
 ]

enter image description here

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