contrast.md

Two colors provide good color visibility if the brightness difference and the color difference between the two colors are greater than a set range.

Color brightness is determined by the following formula: ((Red value X 299) + (Green value X 587) + (Blue value X 114)) / 1000 Note: This algorithm is taken from a formula for converting RGB values to YIQ values. This brightness value gives a perceived brightness for a color.

Color difference is determined by the following formula: (maximum (Red value 1, Red value 2) - minimum (Red value 1, Red value 2)) + (maximum (Green value 1, Green value 2) - minimum (Green value 1, Green value 2)) + (maximum (Blue value 1, Blue value 2) - minimum (Blue value 1, Blue value 2))

The rage for color brightness difference is 125. The range for color difference is 500.

http://www.w3.org/WAI/ER/WD-AERT/#color-contrast

example.js

function getContrastYIQ(hexcolor){
    var r = parseInt(hexcolor.substr(0,2),16);
    var g = parseInt(hexcolor.substr(2,2),16);
    var b = parseInt(hexcolor.substr(4,2),16);
    var yiq = ((r*299)+(g*587)+(b*114))/1000;
    return (yiq >= 128) ? 'black' : 'white';
}