not grayscale, but "brown-scale"

I've seen a graphic technique used and wonder how it's done.

Take a look at www.fivestar.com.  The pictures in the header (to the right) 
are brown-scale, if I can call it that.  Assuming these started as color 
pictures, how can I make them as they are on this site?

BTW, I'm a Photoshop novice.

++Dan

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