Turn light sources off?

[Gunmad]

I'm having a bit of a problem:

I've replaced textures with solid green texture for editing, but because if player's light or other sources of light some parts of these textures (say, a wall) are darker or brighter then others and I can't key out different shades of green reliably.

So what can I do with light sources? I've seen screenshots where people modified lightning and textures with models didn't seem to have any shading on them, but I don't know how it was done.