Does it have something to do with the calcium chloride/coagulant? What happens if you don't add a coagulant?
Here is one explanation, googled it I wondered myself:
Edamame is green because it is fresh off the plant. You can buy it frozen and it tastes soft and delicious as a vegetable. However, it becomes light-colored with age as it turns into a seed. This dry beige form of the soybean is then used for soymilk and tofu. It is usually first soaked before use which whitens it up a bit.
SKK, answering a question about shiso, led me to a website that may help here:
This link is to part 1, about making soymilk, and part 2 is about making tofu. You will see in her instructions and photos how white the soybeans get when they are processed. She refers to 'white soybeans' which I think is to eliminate black soybeans (and maybe there are others too, beyond what I consider tan).