Image by Cody James.

 Understanding a little about how humans perceive color is crucial to seeing impossible colors. Our eyes use something called opponent process to work more efficiently. This plays upon the fact that the eye's primary light receptors, the cones, have certain overlaps in what light wavelengths they can perceive. To save energy, our eyes measure the differences between the responses of various cones rather than figuring out each cone's individual response.

We long ago found out that there are three opponent channels: red vs. green, blue vs. yellow, and black vs. white. (Technically, black and white aren't colors, and their opponent process has more to do with brightness than anything else.) Now, let's say you stare right at the bluest object you've ever seen. Your cones that primary perceive the blue wavelengths are going to be excited, while the cones responsible for yellow will be inhibited. If you then switched to looking at the yellowest thing you've ever seen, the exact opposite would happen.

It probably isn't all that shocking to point out the cones can't be excited and inhibited at the same time. That means that it's impossible to see an object that's simultaneously blue and yellow or red and green. I'm not talking about what happens when you mix those colors and then look at them - obviously, you'd get green and a sort of murky brown if you did that. No, what I'm talking about here are colors that are equal parts blue and yellow at the exact same time. Can you imagine that? Well, you shouldn't be able to, because that's an impossible color.

 This might all seem a bit abstract, but there's some evidence backing up the existence of such colors. A 1983 experiment featured a special machine which separated the fields of vision of the test subject's eyes. One eye would see a red screen, while the other would see a green screen. Given time, the colors would mix together, but the mixing only occurred in the brain. Without the eye there to mediate the mixing, red and green didn't become brown - they became a new color, a reddish-green color that none of the test subjects had ever seen before, and that includes an artist with an extensive knowledge of different hues and shades.

Admittedly, the methodology of that experiment has since been criticized, and many vision researchers say impossible colors are called that for a reason – they really are impossible. There are, to be sure, a lot of alternative explanations for the colors the people saw: they were just intermediate colors between the two, the experimenters hadn't properly controlled for luminance and that threw off the results, or the test subjects were really just see red, then green, then red, and so on, and never actually viewing them simultaneously.

These are all fair points. However, if I may make a counterpoint, you're ruining all the fun, vision experts. Sure, impossible colors might actually be impossible, but that doesn't change the fact that test subjects saw colors they had never seen before. Impossible colors might not exist, but if it's possible to fool our brains into thinking they do, then I'd say that's still pretty awesome.

 This is one of the least scientific viewpoints I've ever put forward, and I'm not exactly proud of it, but hey...impossible colors are cool. Now relax each eye on these two plus signs and see if you can't make some impossible colors appear. Let your eyes cross so that the two pluses are right on top of each other. I'll say right now that not everyone is going to be able to see these weird colors - I'm almost certain that I can't - but I'd still say it's worth a try.

But what if you had a color that only created a response in the medium wavelengths? In real life, this can't happen, as anything that excites the medium wavelengths is going to excite one or both of the other wavelengths. But if you did have a color that only excited the medium, green wavelengths while leave the other two types alone, then you'd be able to see a color greener than any real green.

So that's the theory - here's how you do it. Again, you've to be smart about your opponent processes. If you want to see an imaginary green, you need to find an example of heavily saturated red and one of a heavily saturated green. Stare at the red color for as long as you can, then switch to looking at the green. The red receptors have become too fatigued to do their job and be inhibited by the green color. That means your green receptors are getting excited with nothing to counterbalance them. The result is the greenest color you've ever seen, one that can't exist in the physical world.

Again, this might all seem a bit out there, but America's most lovable evil geniuses have known about this for years. Walt Disney World took advantage of this effect in their design of the EPCOT park, making the pavements a particular shade of pink that tires out the red receptors and forces the park's grass to look greener than it really is. On second thought, I'm not sure that makes this seem any less out there.