They only have a certain number of paint buckets/robots, lets say 10. They have to pick 10 colors that they want to spray all the cars with. If they want to change a color, they have to stop painting cars altogether, clean out the lines/containers of the old paint, and hook in the new paint supply.
And since the same robots are painting other cars currently, they have to finish up orders for the existing colors before they can swap in the new colors. I'm guessing that the delays in Impala production due to GM's financial issues set back their plans to put CGM in the system. I know it's been stated that the Impala uses the same paint line that the Camaro will use (although there could be other cars as well). If you look at the Imapala color options
you'll notice a lot of overlap in the colors. They'll probably be replacing one of the colors (probably Dark Silver Metallic) with Cyber Gray Metallic. They likely already had orders queued up waiting to be built in whatever color will be getting the axe and since those orders have already been accepted into the system, they can't just not build them.
In the end (this summer when summit white is in), 7 of the colors will overlap, leaving 3 unique for each car. So my guess is that they actually have the capability of spraying 13 colors at a time; shared by Imapala, Camaro, and maybe others.