Try opening both the original and the retouched image. Drag the original image from the layers pallette to the retouched image while holding down the shift key (so that is centered). You will now have a new layer in your retouched image's layers pallete. If this layer is not the top layer drag it to the top. Then change the blending mode of the new layer to color. Now you have brightness of the retouched image and the color of the original.
The only problem I can see is that if you increased the contrast in the retouched image too much you won't see the color because really-really dark purple is black and really-really light yellow is white
To show the color and lower the contast a little you could add a levels adjustment layer. Drag the bottom triangle sliders in a little (this is what you would get if you could drag the top sliders out further than the ends ...
Hope this helps, if not just wave ...
Roger