Are there white filaments, that are not so translucent? I find that white filaments are quite translucent and printing 5 layers of white filament onto 2 layers of black filament (at 0.2 mm layers, the white layers being 100% infilled and the underlying black layer covering about 85% of the whole area) produces a slightly grey color on the top.

Is that a limitation of the white colour (or the actual material used)?

Are there materials, that address this issue to some extent?

Adjusting layer thickness while keeping the overall height won't change things, right?
Most of the plastics used for filament are inherently transparent and largely colorless. Color is achieved by adding pigment. Translucent and transparent colors are easy, but opaque colors require large amounts of opaque pigments, and even then, their opacity has its limits - even with black filament!

You can probably find very opaque white filament, but beware - too much pigment will affect the raw properties of the filament itself, either in terms of print temperature or things like layer adhesion or stringing/clogging etc.