I want to give my 1/76-scale model of a Pz.Kpfw.III Ausf.G (late) a winter whitewash camouflage, so what would be the most appropriate base colour (dunkelgrau or dunkelbgelb)?

Well, in all likelihood they would be gray, since they ended production in early 1941 and Dunkelgelb didn't start seeing use until two years later. So even if some G's survived they would likely have been upgraded as appropriate before being sent back out with their newer brethren and wouldn't look like what's in your kit at that point.

But I can't be 100%; I can only state personal opinion that the odds are in favor of dark gray.

Many thanks ... dunkelgrau it is then.