Two possible reasons.
Theme Tweaker has an option to convert your color scheme to grey scale. The algorithm used is this: for a color rgb = (r,g,b), find y = 0.3r + 0.59g + 0.11b, grey = rgb(y,y,y). I am thinking of other color tweaking options such as increase/decrease contrast or brightness, convert to sepia and so on. If you have the algorithms for such conversions, would you like to post them?
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