I've been playing with color contrast for a bit, but I'm not sure if it's just my eyes/monitor that is biased:

Which one of these is easiest/hardest to read for you? (most/least contrast)

@Typeface They are more or less the same to me as soon as I look at them separately and not side by side. Seems that there is some simultaneous contrast at work here.

@larswinter surrounding colors might indeed influence this. I use dark mode everywhere, so the lightest tile is much brighter than anything else on screen.

But 5 still appears to have considerably less contrast for me even if I view it separately.