The oil in pumpkin seeds has a very rare quality. In a thin layer, it's green. In deep pools, it is red. Pumpkin seed oil is one of the most dichromatic substances in the world.
Pumpkin seed oil has a nutty taste, a thick body, and a host of people claiming that it healthies up prostates and cures insomnia. It also has one impressive proven quality: dichromatism. In salad dressings, one of pumpkin seed oil's more popular uses, it adds its own bright green color to lettuces. When it's in a bottle or a deep bowl, however, it's red.
Often the thickness of a material can make a difference in the intensity of the material's color. A light blue pane of glass becomes darker as it becomes thicker. It's very unusual, though, to have a material change color so completely. No matter how much green gets stacked up, it remains green.
In this case, the color isn't being stacked. It's being absorbed. All materials have an absorptions spectrum; a wide range of colors – or wavelengths of light – that the material absorbs. Humans can only see the colors that it does not absorb. They bounce back and become the color we see when we look at the material.
Pumpkin seed oil absorbs many different wavelengths, but it absorbs two particular spectra at a lower rate. It absorbs a wide range of green light at a fairly low rate, and a specific shade of red light at a very low rate.
Imagine a lake full of fish – all different colors, all different shades. In come some irresponsible fishers with wide nets. They trawl the lake and catch almost all the fish. It turns out that, forty percent of the time, all the shades of green fish are at the bottom where the nets can't reach. And one particular shade of red fish is too small and slips through the net.
After the first pass, most of the fish are gone. This includes all the red fish, except that one specific shade. Meanwhile, forty percent of the green fish are still tooling around. So if we were to look at the fish in any section of the lake, we'd see a large group of green, with an occasional spot of red. This is what a thin layer of pumpkin seed oil looks like. There is a sea of green wavelengths of light shooting out at us, and the relatively little red leftover is lost in it.
Now imagine the fishers come back. They make passes over the lake again and again. Each time, forty percent of the green fish are caught. After ten, twenty, a hundred passes, there are almost no green fish left. Meanwhile, to that one shade of red fish, it doesn't matter how many passes there are. It still slips through a hundred percent of the time. If we look down into the lake now, we'll see almost nothing but red fish.
That's what happens when we look through enough pumpkin seed oil. Every extra layer of oil absorbs a large chunk of green light and very little of a narrow spectrum of red light. The more layers we pile on, the more green light is absorbed and the more red is noticeable. Dichromatism in pumpkin seed oil isn't so much a change of color as a shift in perspective.