/**
 * @author  Jonathan Barronville
 * @example
 *     var color_001 = colorShader( '000000', 255 );
 *     // color_001 === 'ffffff';
 *     var color_002 = colorShader( 'ffffff', -255 );
 *     // color_002 === '000000';
 * @param   {String} color_hex
 * @param   {Number} amount
 * @return  {String}
 */
function colorShader( color_hex, amount ) {
    'use strict';

    if ( ! isHexColorString( color_hex ) ) {
        return null;
    }

    var color_number = parseInt( color_hex, 16 );
    var color_rgb    = {
        'r': ( ( color_number >> 16 ) + amount ),
        'g': ( ( color_number & 0xFF ) + amount ),
        'b': ( ( ( color_number >> 8 ) & 0xFF ) + amount )
    };

    if ( color_rgb.r > 0xFF ) {
        color_rgb.r = 0xFF;
    } else if ( color_rgb.r < 0x00 ) {
        color_rgb.r = 0x00;
    }

    if ( color_rgb.g > 0xFF ) {
        color_rgb.g = 0xFF;
    } else if ( color_rgb.g < 0x00 ) {
        color_rgb.g = 0x00;
    }

    if ( color_rgb.b > 0xFF ) {
        color_rgb.b = 0xFF;
    } else if ( color_rgb.b < 0x00 ) {
        color_rgb.b = 0x00;
    }

    var color_new = (
        ( color_rgb.r << 16 ) | color_rgb.g | ( color_rgb.b << 8 )
    ).toString( 16 );

    if ( color_new.length === 6 ) {
        return color_new;
    } else {
        return ( (
            new Array( ( 6 - color_new.length ) + 1 )
        ).join( '0' ) + color_new );
    }
}