You can tell from the curves that A is hotter than B, because you already know that A peaks near the UV, which is much hotter than B, which peaks near the IR. The B-V color index is a more precise way of saying this. Star A is brighter in B (lower number) and dimmer in V (higher number), so B-V < 0. Star B is brighter in V (smaller number) and dimmer in B (larger number), so its B-V > 0!