If what you want to do is see if it really is "black", you can do something like:
if ([[[colorWheel color] colorUsingColorSpaceName: NSCalibratedWhiteColorSpace] whiteComponent] == 0.0)
// we've got black
If you want to see if the color in the picker is "the same" as any other given color, first try converting to the appropriate color space (and it would be up to you to decide which color space that is, be it RGB, CMYK, white and calibrated or device specific). You may want to also figure out if the alpha component matters (are two colors that have the same color components with different alphas the same color? Depends on your needs...)
