Shift a range of Grays to a range of Another Color

[mholder]

I suggest you change your workflow to masking layers.  I'm not totally sure what you are trying to do, but I think using each color as a layer and then using masks is easier to manipulate than trying to select by color and working in another image.
   
Here is an example of what I mean:

  example.xcf (Size: 9.75 KB / Downloads: 190)
I just changed the color in the top layer...