Aaron Marino of alpha m. gives a good set of ground rules to follow when wearing a men’s v-neck sweater with undershirts.
Ground Rules
If the sweater’s v-neck is regular, wear a tee shirt that doesn’t show. When wearing a sweater with a deeper v, you must wear a undershirt underneath so that chest and chest hair aren’t displayed. The tee shirt should be a standard crew or ring neck tee shirt. The tee shirt should be a lighter shade than the sweater. Don’t wear a v-neck tee shirt since the v-necks won’t be the same v shape.