In an SBS, Eiichiro Oda, the creator of One Piece, was requested concerning the Haki users within the Straw Hat Pirates, and at that moment, there were just three.
D: “This might be out of nowhere, but who in the Straw Hat Pirates can use Haki? It’s bothering me and I can’t activate my Love Hurricane.”
Oda: “It’s these three. Their specialty haki color is in the parentheses.”
Of course this doesn’t mean that they will not power up on the other types of Haki (e.g. After pushing his Kenbunshoku Haki to its peak during his fight against Katakuri, Luffy gained the ability to see into the future to a limited extent. In addition he gained Advanced Busoshoku Haki in Wano).
Simply, they’ll be Masters of each different Haki.
Zoro – Master of Busoshoku Haki
Sanji – Master of Kenbunshoku Haki
Luffy – Master of Haoshoku Haki