Changing colors in the scene without re-rendering

[kesonmis]

See this thread:

Get layers/masks of shadows, reflection and other interactions with light

Yes, that was it, I thought you need to map new textures to plane so that they follow the curvature, for this you would need texture coords based mapping. Regarding getting a bit darker image in GIMP, check that you use 32bit float project. 8- or 16bit integer projects can introduce precision issues and can also clip your data. MartinZ, in PS you would have to probably build a plugin, I’m not sure there is a tool to achieve it. Might be though, I don’t know all the ins and outs of PS.

I described the exact same solution as Lukas wrote.