> Please load the attached code (any Emacs version), and run M-x contrast-
> compare with various cutoff values on your favourite system, to find what
> value is best for readability of the colour names (first column).
> 
> Colours with luminance (last column) below the cutoff will be use white text,
> others black. It is mainly Emacs in graphical (non-TTY) mode that is of
> interest.
> 
> Please reply (to emacs-devel or to me) the following pieces of information:
> 
> * the cutoff value you found optimal
> * system information: window system, screen, anything you think is relevant
> * whether you use a light or dark background in your Emacs


Emacs 26.3, MS Windows 10, fairly old Dell monitor.

With emacs -Q, so light background: 

Cutoff 0.325 (default) is OK.
.2, .4, .5, and .6 are all OK. 

To me, showing the same color in both black & white,
next to each other, was the most helpful for deciding.
I looked mostly at the grays, and looked at their hex
codes, which are shown in both black & white.

Based on that, I'd say that that gray60 is about the
crossover point: black or white foreground are equally
readable for gray60 and immediate neighbors.  And that
corresponds to your default cutoff of about 0.325.
See attached screenshot.

Around that area, white text was generally more
visible/noticeable, but it was less _readable_.

But it makes no difference, with that comparison,
whether the frame background is light or dark.  Why?
because the areas to be compared provide their own
background - the frame background is seen only with
the rightmost (decimal-number) field.