Hard - if not impossible - to say.
I've got quite a few cows in the herd with the Simmental color dilution gene, courtesy of several AI sires we used back in the 1980s... in the 'black-factor' cows, they range from dark chocolate brown to mousy brown, to gray to silver to white.
Also have one that's descended from a little yellow line-back Char-X cow we bought in 1986... both she and a white SM-influenced(3/4AN) cow have produced 'white' calves when bred to Angus, Shorthorn, (and even Braunvieh) sires... but also black and dark red calves when bred to SH sires.
So... the Simmental color dilution gene and Charolais color-inhibitor gene are both in the mix here, but other than knowing the ancestry behind those two 'white' cows, I can't necessarily differentiate between the effects of the two.