White cast and grey cast are not the same problem — how do I tell them apart?

White cast is scattering; grey cast is usually hue. They take different fixes, so diagnose before reformulating.

White cast comes from visible-light scattering by particles whose size is comparable to the wavelength. Reduce it three ways: a smaller primary particle — 15 nm scatters less visible light — a lower loading, or better dispersion. Dispersion is the one most often skipped: an agglomerate behaves like a much larger particle, scatters hard and scatters unevenly, so a badly dispersed batch looks whiter than the same formula dispersed properly.

Grey cast on skin is a hue problem. The film scatters and the skin tone underneath shows through unevenly, so the result reads ashy. The standard answer is a trace of iron oxide — yellow and red, at fractions of a percent — to bring the film back toward skin tone, which is what most Japanese sunscreens do.

Two things to check before reformulating: that the dispersion really is good, measured by fineness and microscope, and that the assessment was made on a range of skin tones rather than one panelist.