You probably want to shift hue, but not saturation or value.
So, convert your RGB value into HSV, add 180 degrees to H (modulo 360 degrees of course), and convert back to RGB.
function contrastingColor(color)
{
    return (luma(color) >= 165) ? '000' : 'fff';
}
function luma(color) // color can be a hx string or an array of RGB values 0-255
{
    var rgb = (typeof color === 'string') ? hexToRGBArray(color) : color;
    return (0.2126 * rgb[0]) + (0.7152 * rgb[1]) + (0.0722 * rgb[2]); // SMPTE C, Rec. 709 weightings
}

http://juicystudio.com/article/luminositycontrastratioalgorithm.php
http://www.splitbrain.org/blog/2008-09/18-calculating_color_contrast_with_php
http://stackoverflow.com/questions/301869/how-to-find-good-looking-font-color-if-background-color-is-known
http://search.cpan.org/~ian/Color-Scheme-1.02/lib/Color/Scheme.pm
    http://colorschemedesigner.com/
    http://stackoverflow.com/questions/97646/how-do-i-determine-darker-or-lighter-color-variant-of-a-given-color
    http://stackoverflow.com/questions/635022/calculating-contrasting-colours-in-javascript
    http://stackoverflow.com/questions/141855/programmatically-lighten-a-color
 http://www.mjijackson.com/2008/02/rgb-to-hsl-and-rgb-to-hsv-color-model-conversion-algorithms-in-javascript
 http://www.lighthouse.org/accessibility/effective-color-contrast
http://weblogtoolscollection.com/archives/2009/04/10/how-to-highlight-search-terms-with-jquery/
https://developer.mozilla.org/en/DOM/window.getComputedStyle