These aren't really 'prototypes' per se. They are actually sort of mock up art on what Jazwares might have liked to do with their 3.5 Metal Sonic figure. They aren't even real figures, they're just a photo of the figure, but edited so it looks like it has new paint.
The left one is 'paint-less' with an exposed metal finish and rivet/seam details. It is meant to resemble (but doesn't really) Sonic 2's "Silver Metal Sonic". Remember, he was the very round-headed one with the table-saw blade installed in the back of it. The right is a new yellow and black (with a bit of racing stripe motif) color scheme. It is meant to look like Metal Sonic 3.0 (Mach 3.0) from Sonic Rivals 2 (for the PSP) however, there are several paint details & body details that don't match the game. They have admitted that Sega won't let them produce actual toys of these in this format, so they will always remain in prototypes. (Likely because the lack of real resemblance to the in-game sprites--they'd have to make new figures to do them) Additional info by Aquaslash