here's a new Fuse hot off the press. It's easier to show a picture than to explain what it does:
and another one:
By default it uses a nice rainbow gradient and combines the images in a way that adds up to the original when no transformation is done. This is perfect for chromatic aberration effects. Or to create light spectrum lens flare effects from your image's highlights (linear gamma works best, the Marcie examples were done this way).
It has screen or lighten modes as well and supports all kinds of gradients. But I haven't fully explored use cases for those. It might have a lot more uses than what I designed it for so if you have suggestions or think there's a bug, feel free to let me know.
(I'm secretly afraid that there's already such a plugin in Sapphires or Krokodove that I didn't know about... )
edit: updated version 1.2 fixes the aspect slider, adds x/y size and gamma correction