This will only tell you what color depth your graphics card driver is reporting to the browser. All the MBP screens are just 6-bit panels which dither to get the required color combination.
Oh thank you, I din't know that. I tried it on a 1999 PC (!) and got 32 bit, on a 2009 Mac mini with a crappy display getting 24 bit, and 24 bit also here on the 15" MBP. I have the same panel as you, 9CBB or LP154WP4-TLA1 by LG Philips.
Are you quite satisfied with your display? Or maybe are you going to exchange it hoping to get a Samsung?
I'm asking another test to everybody: please tell me if in this picture http://www.lagom.nl/lcd-test/gradient.php#gradient-h.png you do experience any banding, especially in the areas fading to black. I do.
As I wrote yesterday, I also see it within the Exposé "spotlight shadow" if on a solid dark colored background. Dunno if it's normal.