The boys are assigned as guards to a visiting noble and he seems really nice.

But only to Athos. To D'Artagnan, Aramis and especially Porthos, he's down right antagonistic. The reason is that the man is incredibly racist and pretty much hates anyone who isn't white. The man spits at their feet, is rude to them and sometimes flat out ignores them, but never hits them.

For ages Athos doesn't realise, because the other three grit their teeth and bare it, thinking the guy is just an asshole and is being one to Athos too.

But after a few days of hearing a few sarcastic comments from the others about their 'charming guest' etc, Athos corners them and they're all surprised to find out that the man is only civil to Athos.
It doesn't occur to any of them that the reason is race until D'Artagnan's hot-headed nature gets the better of him and the noble backhands him, calling him a very offensive and racist term.

Athos is having none of it (and neither are the others, Treville and Constance)

(What he does to the noble is up to the person filling)