Extending jQuery with a function that returns a gradient color based on a hex

Gradient

/*
 * Usage:
 * '#' is optional
 * $.gradient( "#CCCCCC", -0.1 ); // returns a 10% darker hex color
 * $.gradient( "#AAAAAA", 0.4 ); // returns a 40% lighter hex color
 * $.gradient( "NTAF2S", 0.3 ); // returns false
 */

$.gradient = function ( hex, opacity ) {
    if ( !/^#?[0-9a-f]{3,6}$/i.test( hex ) ) return false;

    hex = hex.length < 6 ? hex[0] + hex[0] + hex[1] + hex[1] + hex[2] + hex[2] : hex;
    opacity = opacity || 0;

    var rgb = "", c, i;

    for ( i = 0; i < 3; i++ ) {
        c = parseInt( hex.substr( i * 2, 2 ), 16 );
        c = Math.round( Math.min( Math.max( 0, c + ( c * opacity ) ), 255 ) ).toString( 16 );
        rgb += ( "00" + c ).substr( c.length );
    };

    return rgb;
};