Made it a little more DRY

color-shader.js

/**
 * @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 )
    };
    
    for ( color in color_rgb ) {
        if ( color > 0xFF ) {
            color_rgb.color = 0xFF;
        } else if ( color < 0x00 ) {
            color_rgb.color = 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 );
    }
}

color-shader.min.js

function colorShader(a,b){"use strict";if(!isHexColorString(a))return null;var c=parseInt(a,16),d={r:(c>>16)+b,g:(255&c)+b,b:(255&c>>8)+b};d.r>255?d.r=255:0>d.r&&(d.r=0),d.g>255?d.g=255:0>d.g&&(d.g=0),d.b>255?d.b=255:0>d.b&&(d.b=0);var e=(d.r<<16|d.g|d.b<<8).toString(16);return 6===e.length?e:Array(6-e.length+1).join("0")+e}

is-hex-color-string.js

/**
 * @author Jonathan Barronville
 * @param  {String}  hex_string
 * @return {Boolean}
 */
function isHexColorString( hex_string ) {
    'use strict';

    if ( {}.toString.call( hex_string ) !== '[object String]' ) {
        return false;
    }

    hex_string = hex_string.trim();
    hex_string = hex_string.toLowerCase();

    if ( ! /^[0-9a-f]{6}$/.test( hex_string ) ) {
        return false;
    }

    return true;
}

is-hex-color-string.min.js

function isHexColorString(a){"use strict";return"[object String]"!=={}.toString.call(a)?!1:(a=a.trim(),a=a.toLowerCase(),/^[0-9a-f]{6}$/.test(a)?!0:!1)}