Unlike my stance on the sumac powder, I'm more flexible when it comes to replacing the red onions with white onions. However, I wouldn't recommend using yellow onions, which tend to have a very strong, almost harsh taste. Some Sumac Onions recipes call for soaking the sliced onion in hot water for 5-10 minutes before proceeding with the recipe. This is to mellow out the onion flavor. This might not be a bad idea if using yellow onions since they taste stronger than red onions. After soaking the sliced onion in hot water, lightly squeeze, to release some of their juices before proceeding with the recipe. However, if you use red onions, you can skip soaking them first. Besides, I think the marinade does a nice job of bringing out the sweetness of the onions.