Well, if you want the song sad, I'd recommend against playing Bmaj. Bdim would probably be a better idea, as it's the "in key" chord of Am which is the key you're most likely playing judging by the chords you've cited. You can also play Cmaj, Gmaj, and Fmaj. You don't have to limit yourself to those chords though...in fact, now that I think about it, Bmaj would sound kind of cool.