(PHP 3>= 3.0.2, PHP 4 , PHP 5)
imagecolorresolve -- Get the index of the specified color or its closest possible alternativeThis function is guaranteed to return a color index for a requested color, either the exact color or the closest possible alternative.
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 imagecolorclosest().