Most likely dark red, though I would be perfectly happy with clear, other, a mix & match of clear and colors. If it is distinguished somehow from the general production units that is a bonus.
I suspect that there may be less flexibility in truly custom case colors than what the community in general believes. The materials, setup and production costs for my glow in the dark case idea would have quickly cost more than I spent on the Prototype. Given that, I suspect that most prototype cases will fall within the colors voted highest in a poll many months ago. It makes sense though. Prototypes are about testing - and case color & finish is one of those things to test.
So - I'm good with just about anything but pink for the case. I'm even good with a crazy quilt like mix & match of 2-10 different colors/types. (outer lid, inner lid, keyboard bezel, base, battery door, stylus & 4 triggers) could all be different. I'm not sure when I'll find out exactly what comes out of it - may not be until I open the box.
Some people seem to think that the prototype purchase units would need to be visually 'perfect' like those cars used in motor shows for announcing new models. To me, the case is a functional box to hold and protect the delicate electronics within it. It doesn't need to be cosmetically 'perfect' when I receive it because I'm going to use it. Dimension and structural perfection are, to me, more important than aesthetic finish for this.
I should put up a picture or two of my 1Ghz Pandora - it is showing it's miles. I re-cased my old CC unit, so it is actually in better shape. I could re-case my 1Ghz, but I hate dealing with the video cable twist through the hinge - it's a pain to get right.
It is odd to me that people freak out if their new phone has the slightest blemish in the corner of the back cover piece. To me that just gives it individuality. I don't purposely damage or mar my devices, but I don't cry if non-important bits get worn, scratched or dented.