Cyan is a great subtractive primary because unlike blue it absorbs in only one third of the spectrum (the red or long wavelengths). Now let’s see what happens when we mix cyan and yellow dyes. Interestingly, if you look at the artist John Lovett’s page he explains that to mix a yellow and blue you should use a yellowish blue (and a bluish yellow). The main reason for this is that the blue is absorbing too broadly.

You would get a dark desaturated murky dirty greenish colour. However, you certainly don’t get a lovely bright green as shown in the colour wheel with red, yellow and blue primaries. You would get black if the pigments were ideal but they are not. The consequence of this is that you don’t get black if you mix blue and yellow. The same is true of a real blue colorant it does not absorb perfectly at the middle and longer wavelengths. It reflects and absorbs to some extent all the wavelengths but it absorbs more at the shorter wavelength and absorbs at less the middle and longer wavelengths. Notice that with a real yellow colorant, it does not reflect perfectly in the middle and long wavelengths and it does not absorb perfectly in the short wavelengths. (The reflectance factor, by the way, is the proportion – or per cent – of light that the colorant reflects at each wavelength.) Here is a reflectance spectrum for a real yellow pigment. In practice, if we measure the reflectance spectra for blue and yellow pigments they don’t look like those ideal ones I showed above. The problem is, the people who say that blue and yellow make black are wrong of course. Yellow is a good choice of subtractive primary because it only absorbs in one third of the spectrum. Note that blue is a particularly bad choice of primary because it absorbs so broadly across the spectrum.

The people who say that yellow and blue make black are saying so because of this argument. The problem here is that the blue and yellow pigments (between them) absorb perfectly across the whole spectrum. The yellow pigment reflects light perfectly in about two thirds of the spectrum (and absorbs perfectly in the other third). The blue dye reflects light perfectly in about a third of the spectrum (and absorbs perfectly in the other two thirds). The figure below shows what happens when you mix an ideal yellow dye with an ideal blue dye. Well, we need to understand a little science to get to the bottom of this. And you will find other answers that say that yellow and blue make black. So some people say yellow and blue make green.

I want u to color me blue