This is the Latin saying from Gus's door. It means "a grape, seeing another grape, changes color," and it might be the best way to summarize Gus and Call's friendship, for better or for worse.
Jake was tolerant of the cowboys but careful to keep himself a bit apart from them. (17.22)
As isolated as Captain Call seems, he has Gus. Jake Spoon seems friendly with everyone, but he doesn't actually have friends.
As a team, the two of them were perfectly balanced; [Call] did more than he needed to, while Gus did less. (16.35)
This is a funny line, and it sums up Call's view of friendship. Call is comfortable giving Gus guff, and he doesn't think anything that he wouldn't say to his friend's face. It's a relationship built on tough love, in a way, but also on total openness and honesty.
It was an odd thing, but true, that the death of an enemy could affect you as much almost as much as the death of a friend. (16.66)
The ex-Ranger's relationship with Pedro Flores wasn't exactly a friendly rivalry, but it's sort of like the Batman and the Joker, or Superman and Lex Luther. What is a person without his other half? Call will experience a similar feeling, but much, much worse, when his actual other half, Gus, dies at the end of the book.
"No," Clara said. "But I'd want my children to know you. I'd want them to have your friendship." (35.32)
Clara and Gus don't just have a romantic relationship; they have a friendship that might be stronger and more important to the both of them than the romance.
"Let's go have a little fun. I always liked stealing Ellie's boys and here I've got a chance to steal her husband." (69.84)
We've seen the kind of friendly teasing between cowboys with Gus and Call, but this is friendly teasing between prostitutes. Sounds like prostitutes and cowboys are pretty similar in the way they sometimes treat each other.
"You're one of a kind, Gus," Call said, sighing. "We'll all miss you."
"Even you, Woodrow?" Augustus asked.
"Yes, me," Call said. "Why not me?"
"I take it back, Woodrow," Augustus said. "I have no doubt you'll miss me. You'll probably die of boredom this winter and I'll never get to Clara's orchard." (96.101-96.104)
Gus and Call remain friends to the end, able to joke with each other even as Gus is about to die.
Gus had died and left the world without taking him with him, so that once again he was left to do the work. (97.52)
Call rags on Gus, even though his friend is dead. Well, Gus might have died, but the friendship hasn't. Call will probably still talk to Gus in his head until he himself buys the farm. Or the pasture? Or whatever it is cowboys buy.
"You don't know how to live. […] I've lived about a hundred to your one. I'll be a little riled if I end up being the one to die in the line of duty, because this ain't my duty and it ain't yours, either." (25.47)
This is the type of real talk that can only occur between friends. We have to wonder if Call remembers this conversation after Gus does die. Gus dies on a cattle run that he only goes on because his best friend does. That fact adds a little more tragedy to Gus's death.
"I lost three boys, Gus. I needed a friend." (86.138)
A friendship works both ways, and Clara sees it as a slight that Gus didn't come to visit her. But on the other hand, she never asked him to. As a friend, should she expect him to read her mind? Or is this something he just should have known?