bug#33799: 27.0.50; set-foreground-color completion shows background colors

[Juri Linkov]

>> I don't know if readable background is needed for set-foreground-color
>> like readable foreground is used for set-background-color.  I think
>> your change to not use readable background is more correct because
>> then the user will see how the selected color will fit with the background.
>> This helps to select a suitable combination of foreground/background
>> colors.  The same way maybe readable foreground should be removed
>> from the completions of set-background-color.  This would allow to see
>> how the current foreground color will look on different backgrounds
>> in the completion list.
>
> I think defined-colors-with-face-attributes probably needs yet another
> argument, to control whether the non-varying component (be it foreground
> or background) is a fixed value or a changing "readable" one.

Yes, another argument (or maybe rename the new argument `foreground',
but I have no idea for a good name and its values) is needed specially
for set-foreground-color and set-background-color that should leave the
default background/foreground color instead of replacing it with readable.