Vocabulario coloquial: cómo dirigirte a tus amigos en España

"tío/tía" is a typical friendly way to address someone in Spain, without implying family relation, roughly like "mate" or "dude". Forms like "señor/estimado" are neutral/formal. See the section on amistad y apelativos: tío/tía, colega, hermano.

With a girl, "tía" is the natural choice. "tío" is masculine; "hermano" is more like a youthful "bro" and usually between guys; "colega" is possible, but a bit less typical as a direct form of address in this kind of greeting.

"colega" in colloquial Spanish is like "mate/buddy", and also means a colleague at work or in studies. "tío/tía" is a way of addressing someone; "hermano" is "bro"; "tronco" is youth slang. See the guide: colega se usa como “buddy” y compañero.

"hermano" (like "bro") is fashionable among young people, influenced by trap/reggaetón. The others are formal or used more jokingly/ironically.

"tronco" is a colloquial way for young people to address each other; the others sound ironic or old-fashioned. There is an example with "tronco" in youth speech in the reference material.

"¿Qué tal, colega?" is friendly and colloquial. The others sound formal or distant.

"suegro" means "father-in-law", not a friendly way of addressing a friend. The others are typical colloquial terms between friends.

"colega" can mean both "mate/buddy" and "colleague / peer". The guide explains that colega se usa como buddy y compañero.