[R] recovering value of scalar from false color map and image

[Jan Theodore Galkowski]

I have a JPEG map of temperature data, coded using a given legend where
blues are cooler and reds hotter.  Is there any preexisting R magic or
package which might let me recover the value of the scalar quantity so
represented given the RGB values at each point in the picture?

After processing, I'd also like to push the results back through the
translate to get an RGB JPEG again.

Thanks for any and all pointers.

  - Jan Galkowski