bug#25890: `color-values` gives wrong value

[Eli Zaretskii]

> From: "mail@yo.eki.do" <mail@yo.eki.do>
> Date: Fri, 10 Mar 2017 08:13:15 +0000
> 
> > The default was changed to 4 to match how Emacs does color
> > calculations internally.  Using 2 will produce subtle bugs, like that
> > one which prompted that change.
> 
> Thanks for clear explanation.
> So that basically means this destructive change was inevitable.
> I guess you are right.

To my defense, they didn't look so destructive when I made them.  This
function has only one caller in the rest of the Emacs sources.

> > > This change actually had impact on several packages including mode-icons, 
> > > and IMO keeping the original
> > > behavior is a better option.
> >
> > I'm sorry to hear that, but you could still use the optional argument
> > to get back the original behavior, right?
> 
> Hopefully. Actually I'm not qute certain how many packages will be affected.

Neither am I.

I'm open to alternative ideas for how to solve the original bug in a
less intrusive way.  I just don't think defaulting to 2 would be such
an alternative, because the underlying issues are very subtle, and if
we continue using 2 as the default, users of that function will never
discover that they need to use 4.  But maybe there are other ways that
leave everyone more happy.