On Thu, Nov 7, 2019 at 1:27 PM Yuri Khan <yuri.v.khan@gmail.com> wrote:

> I thought I’d chime in with some actual color science to back the
> feelings in this thread.

Very nice.  Thanks.

> There is no formula for calculating how distinguishable two different
> text colors on the same background are.

I would say that the next best thing is to calculate the contrast
difference between two colors period.

blue4 vs black is 1.27
blue3 vs black is 1.87

So very good for reading the text, very bad for distinguishing
the colors in two adjacent letters.

This points to a rule for finding the perfect good compromise:
find the color equidistant to both black and white that passes
both tests.  Of all the proposed faces so far, cyan4 seems to
come closest to passing both these bars. But I found DodgerBlue3
passes the "vs white" (4.7) bar and almost passes the "vs black"
bar (4.1).

So I'd propose that color for the GUI default.

On text terminals we could use whatever minibuffer-prompt uses.

João

(of course we could just let styles choose how it highlights completions
from a given face pool, but that's sadly off the table)