My guess is the same as rowdy's. Unless they actually produce some more uncommon colors specifically for this group buy, I don't see them offering anything unusual, like eraser pink spacebars. Unless you ask for 400 spacebars in 6.25x to be eraser pink, and 400 of the 7x ones to be, say, cyan, and then the rest of the orders of whatever they want, the latter would likely be some split of cream and black. I mean it's nice to imagine that it's possible to get a rainbow of their stock colors in a nice proportion, but it seems very unlikely.
I like the idea of using left-over plastic from other parts of this group buy for spacebars (like from the Esc. pack). Not sure if they'd consider it.