Vision is a favorite example of the creationist crowd. "How could something as complex as human vision simply arise from chance?" they ask. My response: Magenta is a myth. Pink is a figment of your brain. You can see colors that do not exist thanks to the truly kludged color-vision system of human beings. Which is exactly as one would expect from the messy process of evolution.
Imagine a rainbow: red, orange, yellow, green, blue, indigo, and violet. Each of these colors has a specific wavelength of light. In a very objective and real sense, these colors exist, neatly arranged in a line along the electromagnetic spectrum. Get much longer than red, and you're into heat—think of a stove coil that slowly starts to glow in rich red hues. Ultraviolet greets you on the other end—think of the violets from a black light.
To perceive color, we have three distinct color receptors in our eyes. Here's the rub: All three color receptors are copies of one original gene, and they have, over time, drifted apart. One prefers red light, another greenish-yellow (the "green" cone), and the third yellowish-blue (the "blue" cone). Since they are copies of the same original gene, these preferences are weak. For example, a red light on your eye turns on all three kinds of color receptors. The red cone just turns on more than the other two. Your brain has to do a bit of math, comparing the signals coming from each of these receptor types, to determine the color of light. If the red cone is firing more than the green and blue, your brain thinks "red." Red and green both firing more than blue? "Orange" says your brain. Mammal brains excel at this sort of processing, waiting for new receptors to arise from mutation. Women, with four different color receptors, see more colors than men. Experimentally add one of the color receptor genes to a mouse X chromosome, as happened by chance during human evolution, and the mouse can see color.
Here comes the problem: If both the red and blue receptors are firing about equally, but the green is quiet, the math breaks down. In between red and blue is green, but your brain knows the green receptor is quiet. So, it makes up a color—something pink or magenta. Color has a beginning and end in the real world, but is circularized in your mind so that the math always works, creating magenta shades as the opposite to green. Don't believe me? Put something magenta on a white sheet of paper and stare at it for at least a minute—enough time for the receptors to start desensitizing. Then turn your gaze to a blank part of the white paper. You'll see the opposite color, a green shadow of the object.
Color vision in humans is a mess. It's a snapshot of evolution in progress, showing how a gene can drift by mutation and be copied, resulting in amazing new abilities. Exactly as Darwin predicted.