I was also looking for an app to simulate my painting scheme, as I am now about to decide on the colors for my whole army. As you might guess I haven't found one, so I've decided to do some work on my own.
I have no skills with programming, but I do have some with photoshop, so I've edited the image below, added shading, highlights and some customization options.
All you need to use it is Photoshop (Elements also does the trick) or any other layer supporting program (ArtRage, GIMP i guess). All colors are on separate layers, so you can just change their hue (Ctrl+U in PS) to get the desired scheme.
I'll also do another version with armor segments colored independently.
An example effect:
So I just need a place to host the .psd file (or can send it via e-mail if someone wants)
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