Because it's a simultaneity of colors bleeding into each other in front of your eyes, across sun & water. In the night you can't see color nearly as well, not because it's dark either, it's because of the effect on your eyes have when having no sunlight. Everything is the same 2 colors at night without light.
Why do you actually think I mentioned piss in my explanation? Do you think the reason piss was it's color really mattered more than it actually being it's color to make my point?
It's hard to explain my chemical explanation on the body without using color.
Purple is a mixture of blue and red, Green: A mixture of blue and yellow.
Your shit turns out Green when drinking Purple koolaid.
You drink blue and red with water, and it stains your shit somehow. Whether or not you ate with the glass of koolaid. Where does the color process take place? I don't know I'm not a good enough doctor to tell you.
But I do know a great majority of shit is brown. :0 b4 koolaid