The Colour Contrast Check Tool allows to specify a foreground and a background colour and determine if they provide enough of a contrast "when viewed by someone having color deficits or when viewed on a black and white screen"[W3C]. The tool will indicate that the colours pass the test if both the colour difference and the brightness difference exceed their threshold. It will indicate that it sort of passes if only one of the two values exceed their threshold. And finally, it'll fail to pass if neither value exceeds its threshold. You can enter a three character value (eg: 036) and it'll automatically convert it to it's six character version.