[chris (OP)]

Interesting question from Neil Denham on Twitter who says:

Why do all the colours of light mixed together produce white, yet all the colours of paint mixed together produce brown?

Thoughts?

[chiralSPO]

There are three types of color mixing, each with its own set of three "primary colors" (the only reason there are three is because we have three types of cone cells in our eyes: red green and blue).

For light, the mixing is additive. Red + Green + Blue = white (because our cones are tuned to these three colors) To be perceived as Yellow, light can either be a single wavelength that is between red and green (which will activate both red and green cones), or it can be a mix of red and green light (which will then also activate red and green cones). Thus yellow and blue light will also make white light.

There are also pigments, for which red, yellow and blue are the primary colors. Pigments reflect back wavelengths minus a small range of wavelengths. Pigments that look red absorb green light strongly. Pigments that look yellow absorb blue light strongly. And pigments that look blue absorb orange light strongly. Mixing pigments together increases the range of wavelengths that are absorbed, thus the more pigments mixed, the darker the apparent color. Brown is very common as a mix because the colors are not quite balanced right (and our eyes are most sensitive to yellow light--brown is really just very very dark yellow). If the pigments are balanced just right, it will look black or dark gray.

There are also filters, which subtract single wavelengths from the spectrum (white light minus blue looks yellow, white light minus green looks magenta, and white light minus red looks cyan). Thus the primary filter colors are cyan, magenta and yellow (CMYK colors are used in printing--where K stands for black).

[Halc]

There are also pigments, for which red, yellow and blue are the primary colors.

Which make brown.

The actual primary pigments for humans are magenta, yellow, and cyan, which combine in theory to make black.  In practice, none of them filter all wavelengths when combined, so color printers use those three colors plus black to do it proper when needed.

[set fair]

Paint (pigment) absorbs some of the light, reflecing less, mixing paints absorbs more and reflects less. The colour we see is what is reflacted , so mixing paint is subtractive, the more we add the closer it gets to black. Mixing light is additive.

[Petrochemicals]

This is one of those school experiment questions, with the 3 bulbs in triangle formation shining on a central hexagon with different pigments on it or something, showing the mixing of light and pigment. Red pigment under blue light looks black or something not purple.

http://learn.leighcotnoir.com/artspeak/elements-color/primary-colors/

And heres a good educational experiment for Neil

https://www.education.com/science-fair/article/colored-lights-effect/