I had and have some experience with estrela mountain dog. being Portuguese i had alot of contact of dogs of that breed. i don´t know CO so well but i think estrela is not so sharp in showing aggression towards strangers. when he is outside is territory is pretty quiet and relaxed dog. these dogs are used in protection of flocks crossing small villages so that kind of aggression is not desirable. he knows what he has to protect and they are very intelligent dogs. to give an example some years ago i was working has a musher for a guy that had a kennel of 30 sled dogs most of them kept in chains males and females. that kennel was in estrela mountain and during winter those dogs would work on a ski resort. there i meet the most impressive Serra i ever saw, he was big and beautiful he had lost an eye during a fight with a wild pig.
he was alowed to roam the place where the sled dogs were setup during all of the year. this kind of kennel did not had walls or fence so anybody could enter the land and he could go whenever he wanted 24/7 the sled dogs were chained in this land and he had full freedom to do whatever he wanted. he knew which people was allowed to go inside of this place, never fought another dog or caused unnecessary problems he would be the first to bark at the approach of strangers of HIS land and would show that they are not welcomed. but would not chased them if they are outside is territory, but he could. the owner lived 2 km away from this land. but he usually sitck around most of the day there, protecting other dogs and all things inside . he never let anyone inside except if the owner or somebody that he knew was present. and you had to be crazy to go inside with him standing and guarding.
he died at the age of 12 years old living a life of freedom. and that was a great dog. they like to bark specially during at night at any strange sound they ear. they are pretty ok with other dogs. you can see a lot of estrela dogs in estrela mountain, pure breeds and crossbreeds in his place of origin in small villages walking around in the streets. so this breed is not so aggressive has others LGD breeds. but it does his job perfectly. i hope i´ve helped. :D