Soy sauce, light or dark, is made from fermented soybean and wheat, water, salt, and some type of a fermenting agent such as yeast or mold. Light soy sauce is thinner and saltier than dark soy sauce, while dark soy sauce is sweeter, thicker, and has a stronger flavor. Dark soy sauce will give these noodles their dark color. The two sauces are significantly different and shouldn’t be substituted for each other.  If you can’t find light soy sauce, use regular soy sauce in its place. But don’t confuse low-sodium sauce with light soy sauce. They’re entirely different.