I assume we are talking about subtractive colour mixing here - that is, inks and paints etc.

There is a problem. You will find images like the one below, that show that red, yellow and blue are the primaries and that yellow and blue make green.

Sometimes this is represented as a colour wheel:

So some people say yellow and blue make green. And you will find other answers that say that yellow and blue make black. How can this be?

Well, we need to understand a little science to get to the bottom of this.

The figure below shows what happens when you mix an ideal yellow pigment with an ideal blue pigment. The blue dye reflects light perfectly in about a third of the spectrum (and absorbs perfectly in the other two thirds). The yellow pigment reflects light perfectly in about two thirds of the spectrum (and absorbs perfectly in the other third).

The problem here is that the blue and yellow pigments (between them) absorb perfectly across the whole spectrum. The people who say that yellow and blue make black are saying so because of this argument. Note that blue is a particularly bad choice of primary because it absorbs so broadly across the spectrum. [Making the blue even purer would only make the problem worse by the way.] Yellow is a good choice of primary because it only absorbs in one third of the spectrum.

The problem is, the people who say that blue and yellow make black are wrong. This is because in practice reflectance spectra for blue and yellow pigments do not look like this. For a start, they are quite smooth. Here is a reflectance spectrum for a real yellow pigment.

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 It reflects and absorbs to some extent all the wavelengths but it absorbs more the shorter wavelength and absorbs less the middle and longer wavelengths. The same is true of a real blue pigment; it does not absorb perfectly at the middle and longer wavelengths. The consequence of this is that you don’t get black if you mix blue and yellow. You would get black if the pigments were ideal but they are not. We live in the real world. However, you certainly don’t get a lovely bright green as shown in the colour wheel with red, yellow and blue primaries. You get dark desaturated murky dirty greenish colour.

Now let’s see what happens when we mix cyan and yellow. We’ll start with the ideal colours.

It’s very nice. We get a lovely green colour. Cyan is a great subtractive primary because unlike blue it absorbs in only one third of the spectrum (the red or long wavelengths). Note that it is precisely because the cyan does not look pure that makes it a great primary - that’s why I get so furious about people saying the primaries are pure colours. The cyan looks bluish-green because it reflects in two thirds of the spectrum and only absorbs in the reddish part. Neither the cyan nor the yellow pigment absorb in the middle (green) part of the spectrum and therefore the result of mixing cyan and yellow is a lovely green. Except it is not quite true. Remember, this is for ideal pigments. Real pigments do not look like that. Refer back to the measured reflectance spectrum for the real yellow pigment. In reality cyan and yellow do make green but the green might be a little less saturated than you may wish for because of the unwanted absorptions by the two pigments in the areas of the spectrum where ideally they would not absorb (it was the great Robert Hunt, who worked for many years at Kodak - for those who know him - who taught be about unwanted absorptions.)

Have you ever seen this happen. Of course, you have. Whenever you use a printer (which typically uses cyan, magenta and yellow primaries) to get a green, the printer is using cyan and yellow to make the green.

Remember those people who say that you can’t make blue because - yawn - it’s a pure colour that can’t be made by mixture? Well, have you ever printed out blue on a printer. Of course, you have. Let’s look again at our ideal primaries and see if we can explain it.

That’s right. Mixing cyan and magenta makes blue. The cyan absorbs in one third (the red third) and the magenta absorbs in one third (the green third) but neither absorb the short wavelengths.

So finally you can see that the best subtractive primaries are cyan, magenta and yellow because the cyan is red absorbing, the magenta is absorbing and the yellow is blue absorbing. And what is more, you know understand why this is the case (rather than accepting dogma). You also understand why there is a relationship between the CMY of subtractive mixing and the RGB of additive mixing.

But don’t be fooled by this lovely subtractive colour mixing diagram. You might not get such lovely blue, green and red colours when you mix real CMY primaries (either on your printer or with inks/paints). Why not? Because of the unwanted absorptions.

If you want to to know more you could do worse that get a copy of Measuring Colour, now in it’s 4th edition, and authored by Hunt and Pointer.

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