Another question brings up the fact that in many countries, ¡Buenas! is used as a greeting (as an abbreviation of Buenas tardes or Buenas noches). In regions where this is the case, what should be said in the morning (that is, when saying Buenos días is appropriate)? Días is masculine, not feminine, so does the greeting become ¡Buenos!? Or is the feminine form used at all times during the day?
Regardless of the time of the day, ¡Buenas! is understood as an abbreviated greeting. Couldn't elaborate more on the exact meaning of why it is used like this, but we have become used to it as a very generic and informal way of greeting.
This is however a very informal greeting, so in any other situation Buenos días, Buenas tardes or Buenas noches should be used to avoid being considered disrespectful.
I live in Los Angeles, USA. My neighbours are mostly older latina/chicana women, they greet me with "buena" all of the time.
It's definitely a shortened, informal greeting that replaces 'buenas tardes' and 'buenas noches'. That being said, in the morning, they do (always) use 'Buenos Dias'. I usually respond with "buena" as well. It doesn't seem to be disrespectful but, that being said, they all know that I am not of latino/chicano origin, so they may be letting me slide. They do, however, appreciate that I try.
I try to use '¿cómo está usted? as they're older, but I do forget sometimes (cómo estás). They don't seem to care, again, it's possibly (likely) because I'm not a native speaker.
I do hear other neighbours (older and/or more friendly with them) just say "¿cómo?" but I don't ever do that (that's me just trying to show some respect/acknowledge that I'm not really good at Spanish/remind them that I'm not really that comfortable speaking Spanish).