I doubt that it will be gray because grey is actually a color. Black and white are not colors. Black is the absence of color and white is the consentration of color. Therefore I believe that the new version must﻿ play to those features. I believe the third game will be Rainbow Version because rainbow is not a color as well. It's a spectrum of colors.
From: IsaacTheInvoker | #007 Why the hell do people think the third version will be the mix of Black and White? Red and Blue do not make Yellow.
Because Red, Blue and Yellow make up all the primary colours, therefore filling their own pattern (I'm ignoring the fact that it was Red and Green in Japan, with the 3rd version being Blue. >.>).
With Black and White, to some it seems logical that you go for the two shades and then the product of both of them, because there isn't another pattern like the R/B/Y example that can be fulfilled.
I don't think it will be grey (too mundane), but similarly, I don't think it'll be rainbow (too contrasting). Hell, with names like Black and White I'm not even sure if they'll go for a third version. It seems unlikely they wouldn't, but I wouldn't put it past them...
*is super mega hyped for Pokemon Black / White* I don't care that this isn't a witty sig. >.>