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Black is also a colour. Monochrome tones are also colour .
Contrast is the light and shadow. Colour depth can be understood as both - balancing of many shades. And the balancing of a single colour gradation. ;) . The depth given by use of any colour to create an atmosphere, is colour depth. Also called Atmospheric Depth. :)
I meant colour depth. But yes contrast too is great. :)

For me colors are something with hue, but I don't want to argue about it. Thanks for explaining your point of view!

cool with me :)