How do HK restaurants keep wok-fried scallops white, with no visible sear marks?

Your scallops are (beautifully) seared on the outside because they are cooked all the way through in a very hot pan. The scallops in the Chinese stir fries stay pale because most of their cooking is done at low heat.

They are likely cooked twice: first they are velveted (meaning marinated and oil-blanched) and then very quickly stir fried to reheat. In more detail, we have three steps:

1. marinate in a mixture of cornstarch, wine, and egg whites

2. blanch in oil at a very low temperature (at most 300F/150°C)

3. make the rest of the stir fry over high heat, and add the meat at the very end of cooking

Unfortunately, velveting is an involved project. It involves an oil-blanch which leaves you with a lot of waste oil. For home cooking, some recipes suggest to replace the oil-blanch with a water blanch.

The scallops in the first two photos were left on the pan, with no movement, to sear on one side before flipping. To achieve that caramelization takes a couple of minutes before turning. In fact, at that point, the scallop is mostly cooked. In the second two photos, the scallop is likely stir-fried in a wok; that is, it is in constant motion over high heat...or it might be added and left to steam for a couple of minutes, depending on the dish. The scallop will cook quickly, and because of the constant motion (or steaming), not be left in the pan long enough to caramelize.