one fing leads . . .
 
 
 

Edge Detect by Hue and Brightness
Edge detect by hue difference and brightness difference (in an OR relationship). Black/grey/white is considered to be a hue, always sufficiently distant from the colour-circle hues, i.e. if black/white/grey is next to a colour-circle hue then just the brightness difference kicks in.
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Line colour
hue difference
°
brightness difference
grey/black at V: lower than
%
Brightness calculation formula:
 

 
more experiments for the sake of my dreams