>> Where does this 0.6 come from?
> It's documented above, "a cut-off value determined experimentally".
I saw that part, but I was expecting a description of (or a pointer to)
the experiment. Since it depends on the actual color, the human
subject, the monitor, and the ambient lighting (i.e. the search space is
very large), I expect these experiments took a significant amount of
work, so I'd expect some trace of it written down somewhere.