We’ve seen the Yonko’s doing some awesome things in the story but we haven’t witnessed their full powers and capabilities so I believe each of the Yonko specializes on one specific haki
Let’s start with
Busoshoku Haki “color of arms”
This is a spiritual armor that makes the user easier to hit other devilfruit users but it also allows you to defend attacks that would cause you harm. Now 2 of the Yonko specializes in this. Let’s start with
The King of the Beasts Kaido
Now it’s said that it’s inpossible to hurt him so Kaido’s body is fully covered with bushoku haki to such a high level that it’s impossible to break it.
This is a high possibility since he is not connected to any devil fruits at the moment and all the Yonko has the ability of using haki at a really high level so I believe that Kaido would specialize in “color of arms haki”.
Charlotte Linlin “Big Mom”
Big Mom is the second one to to specialize in color of arms, she also has unscratchable body but when she was in a psychological crisis her unscratchable skin seemed to have lost its hardness.
So Big Mom’s spiritual armor seems to be lost when she’s in distress meaning that it’s weaker than Kaido’s bushoshoku haki
Haoshoku Haki “color of the conquering king”
1 in a million has the ability to use this and there is only one Yonko who can use this without flinching
Red haired shanks
When he walked towards Whitebeard half the people in that ship fainted and he did this without directly looking at them while Big Mom had to scream to use her conquerors haki
Shanks could do this in his sleep.
Meaning that his conquerors haki is superior to any charecter who could use this, also Rayleigh has to directly look at something to make his conquerors haki work.
He could possibly use this on a admiral since Whitebeard called it a weird aura meaning that it’s different from his conquerors haki and also KO an army at once it would be stronger than anyone in the whole series.
Oda revealed that he could KO 100k people at once
Kenbunshoku “color of observation”
The last yonko remaining is Blackbeard and the reason why BlackBeard specializes in this is because of his past battle with Shanks and his actions so far
Ok now BlackBeard has a special observation haki that allowed him to predict his opponents movements and some events in the near future
• Shanks mentioned that he didn’t let his guard down when he fought BlackBeard meaning that he was able to avoid Shanks attacks and use that moment to give the scar on his eyes;
• appearing at the right moment he was able to make Whitebeard run out of energy so he’d die – this is kind of a long shot but I think he was able to predict the time that whitebeard was gonna die so he could arrive at the right moment;
• he went head to head with ace – I think he knew he was going to win because he was cocky during the fight even though it was a draw he could have predicted the whole thing.
*Theory by King of beasts kaido