I think that the point of the "red craze" is this; red, though less visible
than blue or green under water, is still the color of blood. Whether fish
see it as "red" or black, matters little; they see it as the same color as
blood.
Copolymer fishing line doesn't only have reflectance, it also has
refractance; it is semi transparent. Since it refracts light, as that color
disappears under water, the refractence changes. At some point, the
refractence may become close enough to that of water so that the line
basically disappears. Red line may also flouresce under full spectrum light,
but as the IR/red is diminished, they may no longer do so. That's the way
hi-vis lines work; they emit a color when exposed to white light, and some
of the color that affects them can actually be outside the visible spectrum.
Red hooks, have little refractive properties, but reflect the light that
hits them. As that color diminishes, they get darker untill they lose all
color (black). At that point, they are still visible, but mainly by contrast
with other colors.
Therefore, red line may nearly vanish under water, while red hooks look like
blood. JAT...

Mark