Change the brightness/darkness for a 3 or 6 digit hex color by adding white/black. Input '#' is optional, returns in 6-digit format like so '#ffffff'.

changeLumosity.js

/**
 * Takes a decimal value and changes it by the specified percentage (based off of the distance
 * from 0 or 255, depending on whether the percentage change is negative or positive, respectively)
 * @param  {Number} value - number from [0, 255]
 * @param  {Number} percent - number from [0.0, 1.0]
 * @return {Number} - number from [0, 255]
 */
function updateValue(value, percent) {
    // Absolute distance from reference value
    var distance = percent > 0 ? (255 - value) : value;
    // Add the % change
    return Math.round(value + (distance * percent));
}

/**
 * Change the lumosity by adding the percentage of the distance to white/black.
 * @param  {String} color - color value with or without the '#', should be 3 or 6 digits
 * @param  {Number} percent - decimal percentage of the change, e.g. 0.10 indicates to
 * lighten by 10%, -0.50 indicates to darken by 50%
 * @return {String} - result hex 6-digit color, e.g. '#ffffff'
 */
function changeLumosity(color, percent) {
    // Strip invalid characters
    var hex = color.replace(/[^0-9a-f]/gi, '');

    // 3-digit -> 6-digit
    if (hex.length === 3) {
        hex = hex.replace(/(.)/g, '$1$1');
    }

    // Hex -> decimal
    var num = parseInt(hex, 16);

    var r = updateValue(num >> 16, percent);
    var g = updateValue((num >> 8) & 0x00FF, percent);
    var b = updateValue(num & 0x0000FF, percent);
    var hexOutput = (r << 16 | g << 8 | b).toString(16);

    // Return formatted with '#' and padded with leading 0s
    return '#' + ('000000' + hexOutput).slice(-6);
};