<p>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.</p>

<p>Is that a limitation of the white colour (or the actual material used)?</p>

<p>Are there materials, that address this issue to some extent?</p>

<p>Adjusting layer thickness while keeping the overall height won't change things, right?</p>

This could be caused by under extrusion, often caused by the bed being too close to the hot end / extruder nozzle. You could try to relevel the bed, or change the screws so that the bed moves down slightly. Often when levelling, you want to feel slight resistance when sliding a piece of paper between the bed an the nozzle. You should do this for all 4 corners. Be careful though, you dont want the bed too far away from the nozzle, or problems maybe arise with the print not sticking to the bed.
Hope this helps, Luke.