[Gimp-user] Can Gimp calculate area of a color?

[Steven Howe]

Howdy,
I'm making a mosaic. I've used gimp to
strip out extraneous background artifacts
posterize the picture down to a sixteen colors.
separated the colors onto different layers.

Now I'd like to know either by area (cm, or inch) or percentage of the 
picture each of the colors (so I can figure out how much tile I'll need).

I don't see an obvious function such a calculation tool for this. Is 
there one and if so, can someone point me to it?

Thanks,

Steven