This is mystery to me but my steamed buns came out brown like baked bread (above). Besides, I did not do a great job to sealing the top of the buns as you can see.
I have to assume that the spices in the curry especially the turmeric and mustard must have caused the brown color but as you can see below only surface is brown and the inside is more yellow which is the color of turmeric and mustard.
In any case, my version of "curry nikuman" was pretty good. Although obviously biased, my wife thought this was the best nikuman she has had.
I adapted my version from this recipe. I reduced this recipe in half but the proportion of ingredients especially water in the original recipe was off and ended up using much more water than the previous recipe specified.
Ingredients for buns (made four good sized ones):
Flour: all purpose, 250grams
Dry yeast: 1 tsp
Sugar: 25 grams (I reduced sugar in half)
Baking powder: 1 tsp (I just realized the original asks for baking powder but I used baking soda—they still puffed up).
Water: nearly 2 cups
I combined the dry ingredients in a Kitchen-aid mixer using a dough hook. I mixed at speed 2 and gradually added water until a ball of dough formed on the hook as well as the center of the bottom of the bowl. I removed the dough onto the floured surface and hand kneaded it until it was nicely elastic. I placed the dough in a Ziploc bag (1 gallon) sprayed with Pam, removed the air, sealed, wrapped with towels and let it rise in a warm place for 1 hour or more (until the bulk doubled). I divided the dough into equal sized pieces (I weighed them), formed the pieces into balls and flattened it to a disk. I covered it with a towel and let it rest for 10 minutes (to relax the gluten).
Filling: I used the pork curry my wife made. I just removed the cubes of pork, shredded it by hand and using a fork, I added enough sauce to make it stick together but not too wet.
I flattened the dough and placed the stuffing in the middle on my left hand (#1), Then I stretched the dough and brought the edge to the center and repeated the process until the stuffing was covered and the surface of the buns were sealed (#2 and 3). I placed the buns on a square of parchment paper and put them in a steamer (preheated, #4). I placed on the lid and steamed for 15 minutes (#5 and 6).
To my surprise, The surface of the buns turned brown looking like baked bread. Again, I have to assume this is due to the color of the stuffing leaching out to the surface of the buns but it is not "yellow" like colors of turmeric and mustard.
In any case, the buns were soft and hot. The curry flavored pork stuffing went very well. This is not quite my childhood "nikuman" but good enough for me and my wife really liked it. The left over buns microwaved very well the next day.