How to make the Custard Bun so white?

There can be several factors here. The first is the flour. Are you really using bleached cake or pastry flour? If you are using all purpose flour, especially if it is unbleached (and bleached is banned in some countries), you will get a more yellowish hue of the dough. This is the most likely culprit. If you are in Europe, try mixing 405er flour (or the local equivalent) with starch in a ratio of 2:1.

The second is the color of the filling. The human visual system does not work in an absolute way, it constructs color relative to all information available. So if there is higher color contrast, and especially more yellow in the filling, the outside will be whiter. The original recipe requires you to use a custard powder with coloring. If you skipped that ingredient, or used an uncolored powder, or your eggs were not as dark as those in the original recipe, then your filling was maybe paler than the original, not creating enough contrast for the bun to appear white. You can add some food coloring of your own if you don't have access to that powder.

The third thing to keep in mind are your expectations. Color gets changed a lot from the capture in a digital camera to your perception on your monitor. Contrast gets changed in-camera and in post production, and most monitors are set up in a way which overemphasizes contrast. It is entirely possible that you would have a different impression of the buns if you saw them in person, than from the picture you are seeing in the blog.