No, other way around. The monochrome mode uses a 1-1 pixel mapping, so it's true 1024x600, but the color mode uses a novel pixel layout that does not have each color component in every pixel:

http://en.wikipedia.org/wiki/OLPC_XO-1#Display_resolution

So, while the resolution always remains 1024x600 as far as software's concerned, the effective resolution is a third when using color.

(Note that the new screen is, I'm sure, not identical to the XO-1 screen, so you can't assume that the Wikipedia description will be accurate for it.)