Returns the pixel value corresponding to the given <code>RGB</code>.
@param rgb the RGB to get the pixel value for
@return the pixel value for the given RGB
@exception IllegalArgumentException <ul>
<li>ERROR_NULL_ARGUMENT - if the argument is null</li>
<li>ERROR_INVALID_ARGUMENT - if the RGB is not found in the palette</li>
</ul>
Returns the pixel value corresponding to the given <code>RGB</code>.
@param rgb the RGB to get the pixel value for @return the pixel value for the given RGB
@exception IllegalArgumentException <ul> <li>ERROR_NULL_ARGUMENT - if the argument is null</li> <li>ERROR_INVALID_ARGUMENT - if the RGB is not found in the palette</li> </ul>