h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

But I always thought yellow, not green, was a primary color, and my computer was weird cause it always wanted things in in red blue and green...

One suggestion that I've heard is that there are two sets of primary colours: additive and subtractive.

CRTs, LCD displays and anything else where colours are mixed by 'adding' in light of a mixture of colours (these kinds of displays all illuminate) use red, green and blue.

Printing, paints and anything else where colours are mixed by 'subtracting' light (inks work by absorbing the light of the colour they are not, and reflecting the rest - they reduce the amount of light that would otherwise have been emitted, hence 'subtract') use cyan, magenta and yellow. Cyan looks blue to a lot of people.

Because colour computer graphics is mostly done on CRTs or LCDs, it's most convenient for them to use the additive system - RGB.

I had made reference to more sorts of ways to describe colour, such as CMYK, but these were edited out of the final version, and I agree with the editors. It was too ambitious, and better to limit the subject strictly to how web-safe colours are arrived at. An article about pigment-based systems and colour for printing ... these would be a welcome addition to the Guide, and would help build a mega-entry on Colour Theory.

I did like all my colour examples, though, and secretly regretted that they were removed. They made the last half of the entry quite gaudy! Also removed were instructions for colouring text, because the PTB don't really like to see it done, and, to be sure ,it is a deprecated tag.

Ioreth: That's a common misconception, largely because the whole light-based vs pigment-based color thing is so confusing. Many people reason that, since you can mix blue and yellow to make green (any 5 year-old with a coloring book knows this), that green can't be a primary color. However, they're not making a true green. To make that, you need cyan and yellow. Cyan is sorta bluish.

When you're mixing pigments to make a color, you need the three primary pigments, which are cyan (bluish, as I've said), magenta (a reddish color) and yellow. However, televisions and computer monitors are all light-based, so you need the primary colors for those... red, blue, and green. It has been determined that the human eye only sees these colors, and by mixing them up a bit creates the rest of the visible pallete. We know this from studies of partial color-blindness, where for a deficiency of one of the three color receptors, people can't tell the difference between magenta and blue, for example, because their eyes don't register the red component of magenta.

Here's the really confusing thing about pigments and primary colors. With pigments, you get primary colors by mixing equal parts of two primary pigments. Magenta and cyan make red. Cyan and yellow make green. Yellow and cyan make blue. I probably have red and blue reversed, but you get the idea. So, how do you make a primary pigment color when using the primary colors in a light-based media? Just as it says in the article... mix two primary colors in equal parts. Weird, isn't it?