Batch convert a range of colors in a file

[dognose]

I would do it in two steps instead, having the shading as a seperate mask.

Then, you'd just change one color in the original image, then apply the mask.

[anthony]

You can also preserve the highlights and shading by using the mask to
overlay the color as a 'tint' either using -tint OR the 'overlay' Alpha Compositing method.

See Tint...
http://www.cit.gu.edu.au/~anthony/graph ... olor/#tint

and Overlay
http://www.cit.gu.edu.au/~anthony/graph ... e/#overlay

I use both of these to good effect in the Advanced IM Examples page
against a 3D shaded greyscale shapes.