Light soy sauce
Japanese soy sauce (usukuchi) that is lighter in colour but saltier than the usual dark kind; "light" refers to the colour, not to lower salt.
Japanese light soy sauce is sold in Asian grocers and by mail order but rarely in supermarkets, and it is worth looking for the Japanese kind, since Chinese light soy sauce tastes different. It seasons broths and vegetables without darkening their colour.
Substitute:
Regular dark soy sauce in a slightly smaller amount, with a pinch of salt, matches the flavour but makes the dish darker.