What is the darkest color out of all of the colors and what is the lightest?

Loosely speaking black and white are, of course, commonly regarded as colours and would be the obvious answers to this question.

Strictly speaking, however, colour is a property of light hitting the eye. Complete blackness is an absence of such light so cannot be said to have a colour.

We surely cannot exclude white so easily. Colour is a property of light dependent on the wavelengths of light received by the eye. If we exclude white because it is a mixture of wavelengths, then we would have to exclude cyan, magenta, pink, brown and so on as these are mixtures of some of the wavelengths. Mind you, if we are talking strictly then we can't use vague terms such as "lightest" and "darkest". Is a low intensity white beam any lighter than the red of a furnace? Scientifically speaking, light can have brightness, intensity, saturation and hue. How would you define "lightness" in terms of these? And that doesn't begin to deal with the quirks of human vision that can, say, distinguish yellow from black than any other colour combination.
It looks like the answer isn't black and white in any sense!

As gmackeematix suggests, it all depends on what you mean, and on what is seeing the light! Some animals see colours only in shades of grey/black/white, and others see certain colours far more clearly. I once was involved in a project to fit bird flight diverters on power lines to protect swans, and it took a fair amount of research to come up with the best colour - a shade of orange. Hence in defining "darkest and lightest" you have to be very specific!