If you’re of the right age to remember classic Apple II games, you likely also remember that many of them had a six-color palette: white, black, orange, green, blue, and purple. Six is not a power of two, which immediately makes it a potentially suspect number when talking about computers. Four colors, eight colors, sixteen colors, 256 colors: these are numbers you might expect to see. But not six.
It turns out that the six-color palette is a consequence of the wonky way that the Apple II produced an NTSC signal. Instead of first encoding colors in terms of their red-green-blue components, the Apple II simply created a television video signal directly.
Simplifying greatly, imagine the color portion of a TV signal as a clock face, with orange at 12 o’clock and its opposite, blue, at 6 o’clock. That puts purple at 3 o’clock and green at 9 o’clock. The clock hand sweeps rapidly around the outside of the dial, and if you turn on the juice while the clock hand is pointing towards a given color, the whole face glows that color. Thus, if the power stays off the whole time, you have black; if the power is on the whole time, you have the average color: white. If you turn the power on only HALF the time, though, you get an average color that approximates one of the colors on the rim of the clock face. Power from 9 to 3 averages out to the color at 12, i.e. orange. Power from 3 to 9, on the other hand, averages out to the color at 6, i.e. blue.
The whole thing is built around the relative phase of the signal; the display of orange and blue are exactly the same, only offset by 180 degrees from each other. That was how the original graphics hardware worked. Purple and green became possible when Apple added an additional way to delay the phase by a quarter cycle: 90 degrees. This is why old TV sets had a “hue” knob that would make the colors all crazy; when you turned it, you were adjusting the relative phase of the color signal, making the colors on the screen follow their cycle from orange to purple to blue to green and back to orange.
Sometimes, I think that computers have gotten just too logical these days. We lost something when hardware became less idiosyncratic. Maybe not something useful or profound, but definitely something quirky and soulful.