If the image is in one of the forms supported by GD, then once you have loaded the image in the usual way, then you can use getPixel( x, y ) to obtain the color value. From the docs:

$index = $image->getPixel(x,y) object method 

This returns the color table index underneath the specified point. 
It can be combined with rgb() to obtain the rgb color underneath the p
+ixel.

Example:

        $index = $myImage->getPixel(20,100);
        ($r,$g,$b) = $myImage->rgb($index);
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