Honestly, I stopped doing to eyedropper method years ago and just started doing it by eye(guess and compare). I doubt there's a real way of doing it other than by eye or the dropper tool, unless Xylax has posted them somewhere.

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If your art program has the option, what i sometimes do is apply a some kind pixel or mosaic filter first, and then use the eye drop from there. That helps break the colours into bigger chunks to aim for

Or as Blackthorn suggested, just eyeball it..

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