Consider a glass of water and a glass of coffee. Their contents differ by no more than a few grams of particles coming from the roasted and ground coffee, yet the former lets almost all visible light pass through, and the latter blocks most of it.
I am wondering what are the most efficient ways to turn water opaque by pouring matter in it, under normal temperature and pressure. By efficient I mean minimising the mass of added matter. By opaque, let's say 99% of daylight is blocked by two inches of liquid.
There is no other reason for my asking than sheer curiosity from the many hours I spend gazing fixedly through the coffee pot.