On 20 March 2018 at 19:17, Eli Zaretskii <eliz@gnu.org> wrote:

> > From: Richard Copley <rcopley@gmail.com>
> > Date: Tue, 20 Mar 2018 19:02:27 +0000
> > Cc: John Wiegley <johnw@gnu.org>, Tom Tromey <tom@tromey.com>,
> >       Emacs Development <emacs-devel@gnu.org>
>
> One- or two-line changes should be okay.
>

Thanks.
Patch 1 is a small fix for the current formula using color-distance.
Patch 2 uses luminance as in Tom's original patch.
I think patch 2 gives better contrast.