bug#15977: 24.3.50; enhancement request: `sample' for Customize colors should show fg & bg

[Drew Adams]

>  > Or, as the bug report suggested, show just the text
>  > "sample", but show half of it as foreground and half of
>  > it as background.
> 
> It might be a bit non-trivial to pick a suitable foreground color for
> "sample" when showing the background but it sounds like a nice idea.

Even with no attempt to do anything special
(i.e., just using the frame background as the
foreground color) it would be an improvement.

And even when the text on the sample wouldn't
be legible/visible (e.g. background swatch
same as or similar to white default background)
it would be useful.

It's a _color_ sample - a simple (background)
color swatch would be OK in all circumstances.
It's just that if we can show its appearance
for both foreground and background then that's
a plus.

FWIW, in my library palette.el I have code
that optionally changes the cursor color to
make it be the complement of the color of
its current position.  Yes, that's a little
bit complicated, but it's doable.

But in the case of this enhancement request
we wouldn't really want the complement.
Probably one of the functions added to
color.el for dealing with color distance would
be usable, to get a grayscale color that
contrasts well with the background.  Just one
suggestion - but see above: just not worrying
about this at all would be fine, IMO.  All we
need is a color swatch (just background, no
text needed).