The author says the first example is slightly incorrect because the Green (a name) and green (a color) have a very different meaning. The example that follows is using the physical color of the building and the conceptual color of the building in some color coded system, which in both cases are arguably colors.

I think the author would probably consider 'green energy' to also be a 'slightly incorrect' example because it doesn't actually refer to the color green. I guess it would be correct if the antonym of green energy were also named after a color.