RE: master 68ae6fa: Improved light/dark colour predicate (bug#41544)

[Drew Adams]

> Thanks to those who replied -- the originally proposed 0.325 luminance limit
> seems to be fairly uncontroversial and can be changed if evidence indicates
> that it was badly chosen. Most of all, the value seems to work well across
> all platforms and a sufficient variety of equipment, which is the important
> part.
> 
> Special thanks to Yuri for helping me confirm that the theoretical 50%
> lightness of luminance 0.18 wasn't necessarily an optimal value in this case.
> Trust my own eyes, but verify!
> 
> The constant has now been given a (provisional) name, for extra clarity and
> ease of adjustment.

Not to belabor this, and I don't recall all that's
involved.

But if there are different eyes, screens, whatever,
would it make sense to use a defvar, or even a defcustom,
instead of a defconst?  Why decide this definitively
at design time?  Does it make sense to hard-code it?

If the question isn't useful, please ignore.