( resource image, int red, int green, int blue )
Returns the index of the color in the palette of the image which
is "closest" to the specified RGB value.
The "distance" between the desired color and each color in the
palette is calculated as if the RGB values
represented points in three-dimensional space.
If you created the image from a file, only colors used in the image are
resolved. Colors present only in the pallete are not resolved.
See also imagecolorexact().