Any artist probably can tell you that when they add yellow paint to blue paint, they almost definitely get green paint.

However, from the earlier post:

Paint is yellow because it reflects red and green light to the eye and absorbs blue.
Paint is blue because it reflects blue light to the eye and absorbs red and green

In this case, by mixing the two paints together, we are suppose to either get paint which:

reflect all three colours, resulting in white paint;
absorb all three colours, resulting in black paint;
or, reflecting and absorbing all three colours in parts, resulting in different shades of grey paint.

So why aint it so?

Not sure if anyone ever thought about this problem, but here’s the reason why:

When people see cyan, they usually think that it is ‘light blue’. In actual fact, cyan has a certain amount of green in the paint itself, making it an ‘un-pure’ paint.

In fact, there is still no such thing as a purely blue paint. By that, it means that there does not exist a dye that would only simulate the ‘blue’ cone of our eyes.

The truth is, our technology has yet been able to find a completely blue pigment and all of our current blue paints actually contains many wavelengths of light around the green part of the spectrum.

Also, apart from those factors, there is also the problem of complex chemical reactions due to the impurities in the chemicals that results in yellow and blue paint giving green.

In fact, according to the theory mentioned in the earlier post,

Green + Magenta = black/white,

Red + Cyan = black/white

Amazing? I was surprised myself. But if you think about it, it does seem to make sense!