Originally posted on Wed, 10/17/2012

Yesterday I posted the following query to my friends:

Okay, everyone, here’s a quick homework assignment:

While you’re commuting tonight and tomorrow morning, look at the “go” stop lights. What color are they, REALLY? Yes, I know they are supposed to be green. But are they, really? And are different ones different colors? How many do you see that match the following colors:

1. Yellow-green
2. Pure green
3. Turquoise
4. Blue

No fair looking at photos of traffic lights, either. This is about color perception with your eyes.

(BTW, I’m partially color blind.)

So here’s what that’s all about. I’m red/green color blind, and green traffic lights have a blueish tint so that people like me can tell green from red. And yet people think of green lights as being a pure green. If you do a Google image search for green traffic light you get a mix of photographs– with blue-green lights– and clip art with pure green. Clearly illustrators aren’t looking at real lights.

This makes a difference to me because I see lots of user interfaces where they use red/green or red/amber/green to display key information: from traffic maps to MacOS window widgets. They never seem to use official traffic light colors. The worst, though, is red/green LEDs on appliances such as battery chargers. There’s absolutely no other context that can be used to indicate the status, since the brightness is identical.

Now, color perception is a tricky thing. It took me years to realize that I really am color blind, because I can nearly always figure out the color from context. (If that sounds weird, consider that you usually don’t notice your disabled color perception when you enter a low-light or colored light environment.) And color itself isn’t in nature– it’s a product of human perception. Consider that an apple looks to be the same color whether you see it under the cool light of noontime sun or the yellow cast of sunset. The light rays that hit your eyes are completely different, but your brain uses the background to compensate in order to construct a stable notion of color. That’s why I specified no photos– the background makes a big difference.

So the results? One person said yucca green, which she described as bluish. (Oddly, that swatch has slightly more red than blue.) Another said there’s variation, and the bluish ones are weird. And another said mostly pure green, with some yellow-green.

Oddly, I’d put the “yellow-green” option in just as a ringer; I never expected anyone to choose it. However, if the background is a bright blue sky, green will look more yellow.

So are green lights turquoise? I don’t trust my color vision enough to say. But you can compare. Blue-green traffic light, turquoise (equal parts blue and green light.)

Here are more details, for the pedantic.

Below is a picture of the official luminance ranges for traffic signals. Modern green traffic signals are allowed to be in the range from this:          to this:         . That’s assuming you’re looking at my color swaths on a pure white background. The ITE standard specifies colors relative to the range of human perception (in CIE 1931 color space)– that fingernail shape in the graph below. Computer displays, which shine only three colored lights, display a more limited, triangular color range with red, green, and blue corners. And you can’t get any of the legal traffic light colors from a computer screen!

BTW, I got my CIE 1931 to RGB conversions here. You probably won’t find a better source, because any serious color expert would point out that you can’t accurately convert CIE 1931 to RGB.

And here’s a mind-blowing example of how language affects color perception.