Reverses RGB colours.

ReverseColour.js

var reverseNumber = function (maxValue, value) {
    return (maxValue - 1) - value;
};
var hex2Dec = function (hexStr) {
    return parseInt(hexStr, 16);
};
var dec2Hex = function (decStr, digits) {
    var hexStr = decStr.toString(16);
    var chars = digits - hexStr.length;
    var count;
    for (count = 0; count < chars; count += 1) {
        hexStr = "0" + hexStr;
    }
    return hexStr;
};
var reverseColour = function(colour) {
    var digits = colour.length;
    return dec2Hex(reverseNumber(Math.pow(16, digits), hex2Dec(colour)), digits);
};