For this look, I prefer monochrome. Mainly, the "flat" effect of it is almost more about the simplicity of it. I guess if they were all blue or all red, it may still have that effect, but varying the colors within the grouping would look cheap.
I agree that monochrome is better. I think the Windows 8 Metro look is not very stylish. If the buttons were all the same color, and a more muted color at that, it would be much nicer.
As I see it, and this is just my personal taste, the buttons should all be the same and muted. I really like black but if MBT and the web interface some day gain the ability to change background color to, say, a very dark blue, I would like the buttons to be in that same color.
Some sort of brushed metal look, perhaps with "indented" icons, might also be cool.