Andy Piper , Jan 17, 2003; 05:08 p.m.
Well, one way would be to reduce the image resolution drastically (on a COPY of the image file!) so that the palm or sole is reduced to a single pixel (or a 3x3 array). In resampling the image Photoshop will come up with an 'average' value from all the previous pixels, which you can then measure with the eyedropper.
Actually you'd probably want the final pixel to be just slightly smaller than the sole/palm, to avoid picking up any color or brightness values, in the 'average', from the surrounding background.
There's also a 'pixellate' filter that will do roughly the same thing (but creating bigger squares instead of reducing the image file size).
In either case you'll have to experiment a bit (with liberal use of the "undo" command) to find settings for image size or pixel size that work best for the image you're starting with.