Hey guys,
just an quick explanation why you should use a green cross-hair instead of any other color.
Except, if you have a green-red colorblindness, than it's an awful idea.
So why a green cross-hair?
The human eye reacts differently depending wich wavelength of light i.e color hits it.
It has been observed that the human eye needs less light of some colors to detect them as bright. Especially green.
That means a green lamp will always seem brighter to you than a red lamp that emits the same amount of light.
That is recorded in the "Luminosity function" of the eye.
The red curve is for visibility in daylight, blue for nighttime.
You can see, that green is pretty much ideal when it comes to "sticking out" from other backgrounds, even other green backgrounds. Ideal for this is a very bright neon like green.
The reason for this?
Well this can not be derived from some specific parts within the eye, but it does make a lot of sense if you look at it from an evolutionary standpoint.
Apes and therefore humans, have always been living in areas with many green plants, of course that is where all the food is. So it comes quite handy if you can distinguish well between different tones of green.
I hope this helps you deciding which color to choose for your cross-hair.
Source: https://en.wikipedia.org/wiki/Luminosity_function
Edit:Spelling...horrible horrible spelling