Dark Soy vs. Light Soy
I'm not sure if most people are confused about this or whether they are aware that in Chinese cooking, there is a "light" and a "dark" soy sauce, but I wanted to demystify this a bit. I think a lot of the mystery comes from the translation of light soy sauce and dark soy sauce. See, in the Chinese name, 老抽 (lao chou) or dark soy sauce actually translates to "old" soy sauce and 生抽 (sheng chou) actually translates to "raw or fresh" soy sauce. The English translations imply color, and yes, the dark soy does carry a bit more weight in terms of color (from the additional caramel coloring that the light soy does not have) and viscosity, but "light" and "dark" should sort of be understood more in the context of flavor and age.
First off, both soy sauces have 3 main ingredients: water, fermented soybean, and salt. The only difference with dark soy is that it also contains caramel coloring. Both sauces state it contains about 66% sodium, which is strange because light soy sauce tastes significantly more salty than dark soy. (If you did what I did, which was to sit down with both soy sauces along w/ a glass of water, this became very evident even after the first try.)
The difference in flavor really comes from the fermentation process. Light soy sauce is fermented less time than dark soy sauce. So as with anything that is aged longer, the dark soy is the one that has developed a more mellow, well-rounded, and mature flavor, whereas the light soy has a bigger kick to it with a slightly younger and fresher flavor. For me personally, I find that light soy sometimes has a slightly sour undertone when paired with delicate proteins like white fish.
Now when do you use what? I do think it's a matter of personal preference. I've heard of the "rules" of when you use dark and when you use light, but in cooking with family, many of these rules go out the window. I do feel that for "red cooking," which is when you essentially cook a meat for long periods of time with soy sauce and the meat picks up the color of the sauce, you do want to keep to dark as that will give you that reddish-brown color. Most of the time, recipes may not even call for light or dark soy, so I mix the portions half-half. Now, I don't think you'll have an issue if a recipe calls for dark and all you have is light, but it's definitely nice to give both a try and explore recipes that call for the different types so that you can try for yourself.