Can someone explain this? electromagnetic spectrum? Help

In my textbook is says

"White light is a mixture of colours, so, if some are missing, the light must be coloured."

I don't understand why the light must be coloured if some are missing.

Thanks

not really a "Physics" issue so much as a perception issue. In physics, "white" is defined as having the entire visible spectrum filled at roughly similar intensities so no one color stands out. So, if that isn't the case, the light cannot be white, and thus the light would be called the dominant color band.

Our eyes and mind perceive saturation of the visible spectrum as "white". All the receptors get signals and that gets interpreted by the brain as white light. Remove some portion of the spectrum, and there is a band of light missing and the brain interprets that as a color and not white because some of the eye receptors are not triggered. there are missing energy bands and thus the light cannot be "white".

The main issue here, it seems to me, is that white means that the visible spectrum is filled, and you want to know why it isn't white when the visible spectrum isn't filled. The answer is, white is what you get when the visible spectrum is filled, and if it isn't, the light is not white, but one of the colors that make up the spectrum. You don't call something that isn't white as white. A dog is only a dog when it isn't a cat. there isn't a "why" to it-that is what the word means.

Not necessarily.

Get a spectroscope, and take a look at a few sources of light, all of which are "basically white" as you would understand white to be.
http://www.xump.com/science/Hand-Held-Sp…

I recommend comparing the following sources of light:
1. a cloud in the sky. (careful not to look directly at the sun, especially through this scope)