Ah, that one is a pet peeve of mine..
For me, the opposite of Birght is Dark. Brigther means more treble (elevated treble freqeencies) whild Darker means less treble (rolled off / attenuated).
Warm, for me, is not the opposite of bright, and is not relates to have relatively more bass than treble (which would be dark). Warm, for me, means having richer, smoother, and fuller bodied presentation (all hard to define, but I hope I'm conveying the spirit of succesfully). It is what many times would be descirbed as musical, pleasant, organic, and sometimes euphonic and even forgiving. There is a frequency correlation in that, at least in my experience, warmer presentation relates strongly to elevated lower mids and upper bass. However, while warm may often mean more relaxed/recessed (see above) treble, it may have great treble detail, whereas darker will have treble rolled off.