GC has what can best be described as a greenish surface glow, almost like a movie version of radioactive toxic waste, but the dominant color really is amber. If you look into a clear glass filled with GC, you'll see what I mean.
In service, the green sheen ultimately fades. For about the first 3k miles, the yellow-green tinge is visible on a white paper towel used to wipe the dipstick. The oil in my sample bottle at the 5k mark still had a greenish tinge on the surface, but it in depth it was deep brown. Now, at 6k, it's dark enough that the tinge on the paper towel is gone. But you can still see a little of it around the inside of the oil filler neck.