Just some further experiments regarding the disappearing green in the vase:

If I leave GC off in the render settings and look at the material this is what I see:

As soon as I turn on GC in the settings and come back this is what I see:

Now I assume this is because the green diffuse color (among others) is being adjusted.

The original color is 0,63,63. Using the gamma formula you get:

((63/255)**2.2)*255 = 11-ish

Which is pretty dark and would explain the gray look.

So I thought I adjust the color in the opposite direction:

((63/255) ** 1/2.2)*255 = 135

So I set the color to 0,135,135 after which I get this:

Now we indeed have some green back, but it is too light. I had a suspicioun that it has something to do with the ambient value being added as it is obviously not the spcularity. And since it was clearly to light this time I've adjusted it the other way around from 0.3 to (0.3**2.2)=0.07 and I get this:

Now this is very close or identical to the original in terms of color. (The specular highlights are softer, so clearly those need adjustment as well.) Alternately I get the same result if I adjust the ambient color instead of the value. e.g. from white to 115,115,115...

What I do not quite understand is why I needed to do the adjustment in the opposite direction to that of the diffuse color???