Colour grading is not a "dead-on science" (although it could be, but that's another debate entirely). Bear in mind that the colour scale is totally non linear, and the steps are somewhat arbitrary in their location. In addition, each grade is a range, so you have L's that are almost a K, and L's that are almost an M. From your descriptions, however, what you have sounds a lot more like an L than a G, and in any case Neil is totally right - what any of us thinks of the stone is totally irrelevant since you are going to get a GIA grade.
Incidentally, since your appraiser has a diamond store, and has suggested you sell the stone at $4500, ask him if he is ready to offer you the 3500 you want for the stone. I would wait to pose the question until he is drinking something. That way it will be much more fun to watch.
Edited by davidelevi, 04 June 2010 - 10:50 PM.