As mentioned above, its all about the refractive index. Red is virtually the same colour as water when submerged.
I think, it's a long time since I did this !, Water has a refractive index of 1, flourocarbon lines are supposed to be around 1.1-1.2, and so are 'Supposed' to be invisible in water?
Charles Jardine did an article in one of the fly mags, about the wavelength,( Colour), in different depths. Red if I remember,( Shortest wavelength ?), went first around 4-6', with White being still visible at the greatest depth.
If you follow the 'ROYGBIV' rule, as ( if I remember prismatic?), Red, Orange, yellow, Green, Blue, Indigo, Violet = White Light).
Colours as we perceive them, are the light that is blocked, and therefore reflected.
Fish do see line shades, (of grey, and more than bloody fifty !

),ask anybody who has fished for carp that have been caught several times !
This is critical in shallow waters in daylight, but less so in deeper water, or darkness, also most sea fish will have never seen line in any colour before !
hope this helps, ???
Mel....