Poo Bare
Ever notice? Poo smells different.
From the same person on different days, or from different people on the same day, or even the same person on the same day but just different times. I'm sure it has to do with what you eat and what's leftover when your body doesn't want it anymore: grains in your poo would be understandably different from ... curry, let's say. (Don't blame me if you're trying to visualize this.)
But over time you can get a consistent understanding for how YOUR poo smells.* And then it can become a decent indicator on how your health is; when I'm getting sick, it's different.
In olden times, the Emperor of China had a (poor) guy whose job it was to smell his poo and determine from it an indication of his good/bad health. (There's a scene about this in the The Last Emperor.) Instead of a toilet, the Emperor poos into a big bowl, which the guy pulls up to his nose and gets a big whiff -- I imagine that he has that look on his face like a distinguished wine taster has when sampling the latest from Mondavi's reserves. (I can also imagine him complaining to his friends about the kind of sh!t he gets from his boss. Sorry.)
While the smell can tell you much, I don't think colour is really a good indicator. Must do more research.
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* I'm just talking about the poo. So you'll need to weed out the smell of the next stall, or the smell of that Glade air freshener, or any of that. Just plain poo.