Maybe I don't understand the question...

It cannot be done: there are more than one RGB combinations that would map to the same 'shade of pale'.

You could of course map the grey values to some -arbitrary- color scheme, e.g. rainbow or sepia.

You can use imageJ to apply a new colour map, but this will only recolour the grey values as the above poster has mentioned. RGB and grey levels have different values, and there are more RGB values than grey levels, therefore the greyscale image will have binned some colours together, which cannot be separated again, so you cannot go back to the original image.