Why does the dumpling /dimsum/momos turn dark brown while steaming

The primary difference between a dumpling and a noodle, besides shape, is leavening. Dumplings usually have either egg or baking powder to make them lighter.

There's considerable bleedover in terminology here. It's reminiscent of the difficulty of defining "chowder". You're always finding a counterexample. I wouldn't be surprised if someone came up with something called gnocchi, gnochs, gnocchetti, that has no leavening. I can think of two myself.

Gnocchetti alla romana are semolina "dumplings" sometimes leavened with egg, particularly when they contain spinach, but more often not. Gnocchetti sardi are dry pasta shapes that kind of look like thin potato gnocchi.

For the reason salt is most commonly used: it's a flavor enhancer. We're not used to thinking of dumpling dough as possessing a lot of flavor--especially when it's there to provide a bland contrast to the savory filling of the dumpling. But salt in the dough will "make it taste more," as my mother used to say. Specifically, you'll taste the subtle richness of cooked flour more easily than if you omit the salt.

The same thing happens when you cook dry pasta. Few amateur cooks salt the cooking water enough. It should be sufficiently salty that some of the salt penetrates the pasta as it cooks and softens, so that by the time it's done, and ready for saucing, its intrinsic flavor--flour and egg--is heightened. Slightly.

In both of these examples it's very easy to use too much salt. When I'm cooking something new and I don't know how much salt to use I try, in tasting, to see if I taste the dish itself better with more salt without actually tasting a salty taste, which indicates oversalting.

Chocolate-chip cookies are so much more delicious when properly salted, as is, believe it or not, blueberry crisp. Generously salting the bread crumb topping of a blueberry crisp brings out the tastes of the ground nuts, the flour, and the butter, and makes it an even more lovely contrast to the cooked fruit.