bug#41544: 26.3; Possible incorrect results from color-distance

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Sun, 7 Jun 2020 11:13:01 +0200
> Cc: Eli Zaretskii <eliz@gnu.org>, 41544@debbugs.gnu.org
> 
> The predicate should work with the argument both as a foreground and as a 
> background colour, for selecting a black or white contrasting colour. The 
> assumption is that the same predicate can be used for both, which may be 
> wrong, but absent evidence to the contrary, I think it is a reasonable one to 
> make.

IME, it is unreasonable to assume that a pair of colors will be
perceived as "contrasting enough" no matter which of them is
foreground and which background.  Background colors affect larger
portions of display, and therefore a bright background is perceived as
brighter and a dark background as darker, than when the same color is
used as foreground.