So, basically, a hexadecimal color is composed of three hues: Red, Green, and Blue. Each hue is assigned a value between 00 and FF, where 00 is darkest and FF is brightest. #FF0000 is pure bright red, #00FF00 is pure bright green, and #0000FF is pure bright blue. If R, G, and B are all 00 (#000000), you get black; if they're all FF (#FFFFFF), you get white. If they all have an equal value somewhere in between, you get a shade of gray. By mixing them up, you get different colors.
Counting in hexadecimal goes like this: 00, 01, 02, 03, 04, 05, 06, 07, 08, 09, 0A, 0B, 0C, 0D, 0E, 0F, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 1A, 1B, 1C, 1D, 1E, 1F, 20, etc.
So, #FF0000 is the brightest possible shade of red. If I want to make it just a hair darker, I can lower the value to #FE0000; this results in a barely discernible difference. Dropping a whole tens-place to #EF0000 is a much bigger change—a difference of sixteen, if I'm not much mistaken.
To darken a more complicated color, you just have to reduce all three hues by the same amount. So, say I've got Light Salmon: #FFA07A. The least I can darken it gives me #FE9F79. That's hardly a difference at all, so maybe I'll try reducing the brightness by five and come up with #FA9B75. But heck, that's still pretty light. Let's go as far as we can while still keeping the hue somewhat recognizable: #852600. That's as far as it can go because Blue is at its lowest possible value. If I change the others at this point, they won't retain the same balance.
And if that makes any sense at all, you can now alter any hex value you like with total precision. {= D But I'm not convinced I'm not rambling incoherently, so let me know if I can clarify anything.
Fun fact: hex colors are made of light, not pigment, so mixing them doesn't work the way it does with paints. Since the three primary hues are Red, Blue, and Green, the secondaries are as follows:
Red + Green = Yellow
Red + Blue = Magenta
Green + Blue = Cyan
That may or may not be helpful, but it's kinda fun. {= )
~Neshomeh
