This was actually something sus back in the days. And people laughed at us when we said zoro has potential for CoC.



I believe Luffy and Zoro also both used CoA for the first time against Crocodile and Daz Bones (Luffy punching through massive amounts of sand, his blood isn't enough to negate that much; Zoro cutting steel).
Luffy, Zoro, and Sanji, even Chopper used Haki Pre Time Skip in some fights; they couldn't control it, and Luffy couldn't damage the Admirals Logia's via Haki because their Haki was much stronger than his (Croc has never been shown as a Haki Merchant, he's a Fruit Merchant, and Luffy also used his weakness, water, to beat him).
IMO this is what Oda means when he says Robin "Doesn't have Haki": she can't control it, but it comes out at times.
Luffy, Zoro, and Sanji all trained with Top/High Tiers during the Time Skip who have Haki. They are in full control of their CoA Hardening, whereas with Robin/Franky and others it only comes out here and there, similar to how Zoro couldn't damage Mr. 1/Daz Bones at all, then suddenly sliced through him.
Same thing happened with King with CoC: Zoro's CoA wasn't enough, so, he unlocked CoC to defeat King. Zoro only cut King with CoC. Luffy + Zoro didn't use Adv CoC during Seraphim fights = no Seraphim damage.
This also explains why Zoro doesn't have CoC listed on his cards/databook shit, and didn't use it against Lucci; Zoro can use Color of the Supreme King at times, but he doesn't have control of it like Luffy does.
Oda's already showed us the Crew that will teach the Straw Hats Haki control, too. Rayleigh and Gyaban don't have Kamusari... who does?

Akagami Pirates will teach the Mugiwara Pirates Haki
On another note, the reason Aramaki didn't take damage from Momo is because fire isn't a forest's true weakness. Forests NEED controlled burns to take out dry vegetation; hell, there are plants that live in sections of the ocean that are incredibly high temperatures that would melt most metals.
ICE is Aramaki's weakness; Ice is what kills plants by freezing the ground that needs liquid water to grow roots. You'll see vegetation all over areas with active volcanoes. See any plants in Antarctica?
Ice Time...
Kuzan freezes Green Bull's balls off in Wano Act 4
Kuzan is Aramaki's natural enemy