@green But CMYK also conveys information that is absent in RGB, with the K component.

K stands for black (don't know why). CMYK(0%, 0% 0%, 100%) would look black.

But actually, we can go more black than that.

See, because every ink subtract one another, if you added the other inks, they would make the result even more black. Hypothetically, setting every ink to 100% would look dark as hell... if only printing it were possible!

Maybe you've noticed that a thick ink drop takes forever to dry. This would happen if you had every inks set to 100%. It would smudge all over and be a disaster.

So we can go darker by setting other inks higher, but we can't go too far. Printing companies will tell you what colour you can use to achieve the blackest colour they can print. This is called a "true black". The printing company I'm working with uses CMYK(60%, 40%, 20%, 100%).

So we have CMYK black, which uses only one ink, and true black, which uses every ink.

RGB conveys none of that information! In RGB, there is only one black, it represents an absolute black which is impossible to have in real life.

If you're curious as to why the distinction matters...

Since RGB black is supposed to be an absolute black, most conversion algorithms will try to represent it with a true black. This is why trying to convert RGB(0, 0, 0) will usually not give you CMYK(0, 0, 0, 100%).

This is problematic, because they aren't actually aware of the printer's capabilities

And also because true black is not always what you want. Since you're printing with 4 inks, you need all 4 colours layers to align perfectly in the final product. This is not always the case depending on the printing method.

You might see a little bleed of colour around areas with true black. This is acceptable for large elements, as the bleed will be small in comparison, but it's completely unacceptable for small text, for instance.