Low flash powders are indeed comparatively low flash. But there are so many variables that there is no flash that is consistently predictable.

Shorter barrels enhance flash as do high pressures. A Sig .357 is a high pressure round generally shot from a <5” barrel, so there is almost always going to be flash in dark conditions.

If this question is related to firing in self defense in dark conditions and trying to retain night vision for accurate shooting, eh. The likelihood of taking advantage of such a flash reduction in a situation that is so remote to begin with is, well, remote. There will be flash. It will slightly effect your vision, but not enough for you to be missing with a few incredibly fast shots at close range.

I think it’s worth it to use low flash powder for the potential of a somewhat lower flash, but if you really need almost no flash because of shooting frequently in the dark, then you might want to try a pistol compensator/flash hider as an additional effective solution.