I use similar foam mats on our small shooting stage--they're almost the same color as our green screen and they work great. Here's a picture of what it looks like:
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We purchased ours from this vendor:
http://wondermat.com/I've never tested white but my guess is that white probably isn't the best choice with either PS3 Eye or Kinect--with PS3 Eye, white may show shadows more easily which could affect background separation quality, and with Kinect the brightness of the surface may affect the IR quality. I could be wrong though.
On other hand, if the lighting in the room isn't very bright to begin with, the bounce on white may be helpful to even out the lighting on the subject. I suggest that before you make your purchase, you could try spreading one or two white sheets on the floor and see how that affects your mocap experience.
Sorry, I hope that didn't just complicate your decision making. :)
For me, the two reasons I got my mat was to extend the green screen across the floor and to give performers a comfortable surface to work on. For the purpose of motion capture, it's not especially necessary. (Unless you plan to fall down a lot--which I do, and sometimes on purpose even.) :)
G.