Returns the index of the specified color in the palette of the image.
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.
An image resource, returned by one of the image creation functions, such as imagecreatetruecolor().
Value of red component
Value of green component
Value of blue component
Returns the index of the specified color in the palette, or -1 if the color does not exist.