brick node: no worry. If you make a wall out of it, and use plain lighting, then dark red (127,0,0) will come out as dark red. When you make a shiny ball out of it, you'll notice that the shadows will not darken that much and the highlights will be softer, compared to a non-GC render.

diffuse node: watch out. Dark red (127,0,0) a 100% will behave different from bright red (255,0,0) at 50%. This is because the (50%,0,0) color is anti-GC'd but the 50% value is not.
Also channel addition (diffuse + ambient + ...) works different under GC. This turns the older Vicky textures - with the cyannic teint in diffuse - into the long smoking zombie look when GC is switched on. Knownto PoserPro users since 2010, now in theatre for P9 users as well. Welcome aboard.

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though