Why is soy sauce black?

Answer 1

Any black thing, including soya sauce, will appear black because it absorbs most of the light that falls on it.

So why do "black things" absorb light? Because their surface or their interior is unable to reflect light. (In other words, "because it does". Why do metals appear shiny and metallic? Because they do!)

Black things will normally appear black in other light; but other colours may appear black in other lights. For instance, a blue object may appear black in orange light, because orange light has little blue within it for the "blue" object to reflect.