Twilight/Dusk/Supersonic blue are all essentially the same: a grayish (steel) blue. SO dull. I'm not sure what GM is thinking here, but I already know of several people who have canceled their orders just based on this decision. Bad idea to eliminate blue...
The instructions for the ZL-1 are, if you have ordered an IBM one, contact your dealer immediately or the order will be rejected. I don't know as of yet how many people who have ordered IBM recently will get theirs... got a bad feeling about that.