GIMP :: Compositing Layers To Simulate Lights

I saw a tutorial recently that describes how to use Photoshop to composite layers and simulate multiple lights. I tried it out in GIMP, figuring the result would be similar, but the image doesn't looks the same in GIMP as it does in the render.

Here's the link: [URL]

I posted as R. Moore, and the creator of the tutorial responded, as you'll see on the page. But I'm afraid I don't quite understand his response and I don't want to badger him with questions like this.

So here's what I do. I'm using Poser and I have three lights: a key light, fill light, and back light. I turn on only the key light, render with that. Then I turn on only the fill light, render with that, then I turn on only the back light and render with that. Then I put them in GIMP using "Open as Layers" and put a layer of black beneath those. Then I turn all three light layers to Screen mode, as the guy suggests, and play with the opacity. But the colors are wrong. Reds too red, blues aren't blue enough...

I get the impression that, done correctly, you can arrange the layers in such a way that they look identical to the actual render, just so long as the layers are in the proper mode. Am I right or wrong about this, and what can I do about it? I would be glad to attach files if necessary.