When I was younger, at least in Madrid we used "colega" very often instead of "amigo" in an informal context. I am not sure if it is still very used or was replaced by another word :-)

Colega is used to called the people you work around with, or are in the same working area like you. Like doctors call other doctors "colegas"and so on. They dont have to be your friend in other to called them "colega" And "amigo" is more general and includes "friendship" and this people are more close to you than your "colegas".

Hello Jin. The meaning is almost the same, but "colega" is more informal than "amigo". And, although it is informal, we don't use it often here in Spain. You definitely shouldn't use it in a formal message. ;) I hope you find this useful.