JS function to lighten or darken a HEX colour.

gistfile1.js

/**
 * Util function for lightening the colour with a %
 * @param - string - colour with leading #
 * @param - number - percentage to lighten by
 */
function lighten (c,p) {
	var n = parseInt(c.slice(1),16)
	, a = Math.round(2.55 * p)
	// Bitshift 16 bits to the left
	, r = (n >> 16) + a
	// Bitshift 8 bits to the left based on blue
	, b = (n >> 8 & 0x00FF) + a
	// 
	, g = (n & 0x0000FF) + a;
	// Calculate
	return '#' + (
    		0x1000000 + (r < 255 ? r < 1 ? 0 : r : 255) * 0x10000 +
    		(b < 255 ? b < 1 ? 0 : b : 255) * 0x100 + (g < 255 ? g < 1 ? 0 : g : 255)
    	).toString(16).slice(1);
};