Thursday, October 22, 2009


Yellow has never seemed like much of a colour to me. It's more like off-white than a separate colour. I want to describe it as a dark white, but at the same time as a bright white. Are dark and bright opposites? I don't think so, because light blue is not bright; but I'm not really sure what bright means, unless it's perhaps "reflective."

This brings me to the question of what distinguishes yellow from gold (and grey from silver). I remembering wondering about this when I was a kid, because Crayola's 64-pack had separate colours for yellow and gold, but I couldn't figure out what mixture of paints would reproduce gold. The trick, I realized, was that gold-coloured crayons had a sparkly element in them that clearly distinguished them from yellow; ditto for silver vs. grey. In practice, gold seems to be a little darker than pure yellow, but that doesn't stop people from describing blond hair as "golden."

Yellow is one of the three primary "subtractive" colours. I first heard of the idea of subtractive colours while sitting next to someone from the printing industry on a flight to Europe. We had a lot of time to talk, and he wouldn't let me sleep, so I got to hear a lot about how printing is done. I was baffled by his description of how printing nowadays is subtractive: a page goes through the printer three times, each one "subtracting" a certain colour. I was envisioning a black page being lightened to white, and his description did nothing to clarify the process to me.

I understand now that "subtractive" is just a term for how we perceive colour in objects. The colour we see corresponds to the colour (wavelength) of light that is reflected back to us; the object absorbs all other colours. If we add all colours to an object, it absorbs all wavelengths of visible light, and therefore we perceive it as black. By "adding" colour, we have subtracted from the amount of reflected light.

This makes more sense if you think about a computer monitor (or a television), where we perceive the light directly. Each picture element actually contains three light sources, one in each of the primary colours. If no colour is on, the element is dark, i.e., black. If all colours are on full-strength, the element shows white light. In this case, when you add a colour, you bring it closer to white; on paper, if you add a colour, you bring it closer to black by subtracting from the reflected light.

Yellow is one of the three primary subtractive colours, but here's the strange thing: in additive colour (i.e., light), the primary colours are red, blue, and green. This is one of those asymmetries in the world that keeps me awake at night. It's like the problem of why objects in a mirror are reversed left to right, but not top to bottom; only I think I've solved that one, whereas I have no idea why the primary colours should be different in additive vs. subtractive colour. It would make far more sense to me if the primary additive colours were purple, orange, and green. I would still have no idea why they would be different, but I would have some hope of understanding it someday. To add to the confusion, the colour spectrum of light places yellow just where you would expect it, between orange and green, and green is between yellow and blue. It would seem perfectly logical that green is formed by combining yellow and blue, not that yellow is formed by combining green and blue. The whole thing is so crazy that I wouldn't believe it if someone told me; but you can actually try combining different colours of light and demonstrate that it is true.

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