String function that converts a hex value (e.g., #FFFFFF) into a percentage between 0 and 1. The darker the color, the closer the value will be to zero. The lighter the color, the closer the value will be to one.

string.color-brightness.js

/**
 * HEX TO COLOR TONE
 * Convert a hex string for colors into a percentage of color tone
 * This function takes accessibility into account when color correcting the brightness differences between RGB
 *
 * @return float(0, 1)
 * black = 0
 * white = 1
 **/
String.prototype.getColorBrightness = function() {
    var c,
        blueMultiplier = .068,
        blueVal,
        greenMultiplier = .691,
        greenVal,
        redMultiplier = .241,
        redVal,
        currColorTone,
        maxColorTone = 255; // 255 is the maximum hue value you can use

    // MAKE SURE THE CURRENT STRING IS A HEX STRING
    if(/^#([A-Fa-f0-9]{3}){1,2}$/.test(this)) {
        // CONVERT INTO ARRAY
        c= this.substring(1).split('');
        if(c.length== 3){
            c= [c[0], c[0], c[1], c[1], c[2], c[2]];
        }
        c= '0x'+c.join('');

        // CONVERT HEX TO FLOAT FOR EACH COLOR
        redVal = Math.pow((c>>16)&255, 2); // value to the power of 2
        greenVal = Math.pow((c>>8)&255, 2); // value to the power of 2
        blueVal = Math.pow(c&255, 2); // value to the power of 2

        // CALCULATE CURRENT COLOR TONE FROM COLORS
        currColorTone = Math.sqrt((redVal * redMultiplier) + (greenVal * greenMultiplier) + (blueVal * blueMultiplier));
        /**
         * GET COLOR TONE PERCENTAGE 
         **/
        return currColorTone / maxColorTone;
    }

    throw new Error('Bad Hex');
};