I don’t understand how black can be added in CMYK color mode. So green is made up of 85% cyan, 14% magenta, 100% yellow and 2% black. But I thought black is created by combining 100% of cyan, magenta and yellow, so where does the 2% come from?
It comes from:
1. The imperfection of inks
An ink is made from available materials, which need to be affordable, safe, reliable, etc. and they need to give reasonable results.
With the current technology, the inks are a good compromise on those points, but they are not perfect. When we combine 100% of Cyan, magenta, and yellow, they do not manage to absorb 100% of the light so the obvious choice to make things look black is to add black.
2. It is a good old friend
Let us add some additional characteristics of black ink. It is probably a good old color we are used to having on a book, since way before they were printed books, manuscripts, ever since cave paintings. It is stable, good for sharp text, etc.
3. It is cheaper in the long run
Black ink is cheaper to produce, so it is easier to use when we actually need black.
So in the current CMYK model when we need to darken a color that is first darkened with some complementary colors (1), we start adding black to the mix little by little until the dominant color on a black image is black ink. Some years ago this was a method of saving costs when reproducing color prints, trying to replace the neutralized CMY colors as soon as possible, which was called achromatic method. Now days is more important having nice vivid contrasting colors, but it is still cheaper adding black.
4. So adding the above, it is simply logic to use it
(1) Answering your second question.
Black starts to replace neutralized colors. When you have two primary colors, Cyan and Yellow; Green in your example, it has the “maximum” saturation for that color (forget for a bit the lightness of it).
When we start adding the last primary color, two main things happen, it starts to darken, and it starts to be neutralized moving it to black.
Adding black too early on light colors will be too abrupt, so neutralizing it with only the complementary is a good option (Some Magenta). When it is dark enough that this black ink does not show a clear border, we start replacing the color with Black.
Modern inkjet printers even have some light versions of Cyan and Magenta, others have lighter versions of black, so these dots and transitions are more subtle in light colors.
Let me spam you with some examples I made. The page is in Spanish but just look at the images. Here it is how a perfect CMYK ink would transition. And here is an example of how black is introduced little by little when color gets darker.