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Well it depends, what software do you use? Personally, I'd make a little Python script that would do the work for me. If you are OK with finding some values manually of course, you can just load the image into something like Gimp and use the colour lookup tool to find the colour of the squares. But as I said, it depends what the context of this is, and how many images (and squares) you want to do it for.
So i take a pic, pixelate it so its roughly 40x40, then I posterize it so it only has a limited palette, but then how do I know RGB is in each of the squares? - ie how do i get a table like:
Row1,Col1 = RGB 85 68 52
Row1,Col2 = RGB 86 68 68
etc