CMYK - Why is White not white, and black not black?

[Merlinski]

I'm sure there's a perfectly reasonable explanation to do with profiles or something and I'm not suggesting there is a problem with the program, just my understanding.

Here's some examples...

Black Values (Yes I acknowledge the difference between screen/light versus print/ink)
RGB R-0 G-0 B-0
HSV H-0 S-0 V-0
HEX 000000
CMYK C-74.1 M-68.3 Y-66.6 K89.8 ?

Why would CMYK not be 0 0 0 100 for example?

White Values
RGB R-100 G-100 B-100
HSV H-0 S-0 V-100
HEX ffffff
CMYK C-4 M-0.4 Y-0 K0 ?

Why would CMYK not be 0 0 0 0 for example?

Thanks for any insight.

Chris

[handrawn]

for technical reasons [basically the black ink is not dense enough to cover large areas by itself] when printing, a rich black is default - ie a mix of one or more % of c/m/y as well as k

that explains why it is not 0 0 0 100 - but why is is the exact % ratio it is I don't know - there is no hard and fast rule, it may depend on colour profiling...

in printing white equates to no colour at all so I would expect 0 0 0 0 but again it may be colour profiling

[gwpriester]

I suspect it is a mathematical flaw. White would be the total absence of any color. As Handrawn pointed out, CMYK printing ink has transparency. 100% black will print black but not a dense black. The addition of a little of the other inks creates a richer, deeper black.

[handrawn]

I'm no printing expert Gary, but I do know, from painting, that what pigmentation suits one kind of 'white' paper, may not suit another kind

[Acorn]

My gut tells me it is because the CMYK Gamut/Colour Space/Profile can not achieve C-0 M-0 Y-0 K-100.
The closest to the blackest black is C-75 M-68 Y-67 K-90. You had C-74.1 M-68.3 Y-66.6 K-89.8.
It is also referred to as Rich Black but people tweak the proportions. Think of it like adding undercoats.

Paper and inks can still let light through but too much may not be a good thing.
Such values are to stop the printed material appearing too muddy; you might think C-100 M-100 Y-100 K-100 would solve everything.
CMYK printing uses industry standards like Pantone Black or you can specify Spot Colours.

I am not authority but I visualise CMYK as brush painting colour onto a surface. RGB is shining light onto it.

Acorn