What do the numbers mean? Briefly, they are based on RedGreenBlue. The first two numbers/letters signify how much red is in the colour; the middle two numbers/letters how much green; the last two numbers how much blue. Remember that the colours being mixed are not pigments but light (like the colours cast from a prism) so the traditional pigment colour wheel does not work.
This is how the colours mix with light.
Red, Magenta, Blue
Yellow, White, Cyan
Green
If all the colours are used fully (signified by the letter "f"), the light becomes white. (Remember when mixing pigment, all colours become black.) If none of the colours are used (signified by the number "0" zero), there is no light and so it becomes black. By mixing red and blue equally, one gets magenta. By mixing Red and green equally, one gets yellow. By mixing Blue and green equally, one gets cyan (turquoise). I'm still trying to get a handle on what the other numbers and letters mean but you can see how easy it is to get the primary and secondary colours now.
::: Red is "#ff0000"
::: Green is "#00ff00"
::: Blue is "#0000ff"
::: Yellow is "#ffff00"
::: Magenta is "#ff00ff"
::: Cyan is "#00ffff"
Black is "#000000" and white is "#ffffff".
I have also made a little colour chart (gif 4.72kb) that you can use as a palette if you are making your own gifs.
