2 colors to make yellow
Answers
Making recourse to the figure above demonstrating additive color mixing, one sees that yellow light is composed of an (additive) mixture of red and green light. When we mix the two paints, the resulting substance has red paint and yellow paint.
Answer:
Yellow happens in more than one way. A nice, pure kind of yellow is 'Spectral' yellow. This can be seen in a rainbow and it's a colour that comes straight from the sun in the form of yellow photons which are those that have exactly the right wavelength to be yellow.
It has been thought in the past that we humans might have a yellow receptor in our eye as we seem to feel very strongly about how nice and lovely the colour yellow is. The fact is that humans do not have a receptor for yellow, but we do have receptors for red and green. Those yellow photons trigger both the red and the green receptors and our brain does the maths in order to understand that the colour is yellow.
That's all about light... when we start talking about paint we all get ourselves in a muddle about primary colours and mixing things. Paint has extra qualities that need to be considered.
First of all paint is a chemical and the two colours that you mix together gain chemical properties from each colour. One of paint's properties is that it absorbs light of different wavelengths in different amounts: that's what makes it a colour to begin with. But this absorbing pattern is complicated - it's not like the RGB sliders on software - paint absorbs all of the different frequencies that you can imagine, all of the visible ones in the rainbow, in different amounts. When you mix one paint with another you can be combining the absorbing profile of one paint with the other.
This is why yellow is important for painting - it absorbs, to put it crudely, blue. But guess what? There's more than one way to make yellow paint. Some yellows might only reflect back spectral yellow, but not red and green. Some yellows might only reflect a bit of red and a bit of green but not spectral yellow. These two yellows will look very different under different light sources and will behave differently when mixed with other paint colours.
So Red and Green light will add together to make yellow, like on a TV or computer. Yellow light exists as a thing by itself, as seen in a rainbow. When painting you need to start with something that has a yellow pigment because other pigments won't combine to create a yellow.