Depending on the two items, it can either look perfectly fine or hideous. Some of my friends often default to a dark thin top under a brighter shirt and it usually looks fine, though maybe that's just because a lot of them have long-sleeved binders and they decided to just integrate. I wouldn't really suggest trying it with anything but very dark colours or white, and white under black is so 90s-to-late-00s skater/probable Converse connoisseur I'd tread carefully to not gain a sudden appreciation for Franz Ferdinand or something.