I normally try to read the entire thread before posting, but often sometimes sticks out and I just need to throw my two cents in…
Preliminary: Despite some bullshittyness - everyone can avoid death, apparently, and we still have the issue of Trebol and Doffy vs. Luffy by himself (unless Luffy can take them both, which is a bit unbelievable due to how swole doffy is, we'll still need another chapter of Trebol being taken out of that fight) - I generally enjoyed the chapter. I can be a bit of a Law fanboy, and seeing him fail at his life's ambition (revenge) despite giving it his all is miserable. And that's a good thing. Plus, goddamn! That last page. Oof.
Okay, so here is what I want to get at:
Trebol is a logia.
As far as we know, logias can only be damaged with haki infused attacks.
So Trebol can only be damaged with haki infused attacks.
Luffy kicks Trebol, but his leg is not blackened, as is usual for haki.
How does Luffy manage to kick trebol?
Well, the difference, I think, is between hit and damage.
So first, here's the obvious possibilities:
(1) Oda just forgot to blacken Luffy's leg.
This has happened before. Typically, these are corrected in volume releases. Since, with time, this will prove itself empirically, I don't think the conversation about it would be interesting. Not ruling it out - but it strikes me as not subject to being "armchair'd", and is in any case boring.
(2) Luffy is using Colorless Haki (usually cashed out as Armament vs. Armament:Hardening); Colorless Haki confirmed.
Many, as far as I can tell, think that Luffy kicking Trebol confirms Colorless Haki is still a thing. Let it be known that I believe Colorless Haki to be a thing. HOWEVER I do not believe that Luffy's attack on Trebol confirms colorless haki. Let me put it this way:
If colorless haki exists, then this attack is possible
However, it is NOT the case that if this attack is possible, then colorless haki exists.
There are other possibilities. It is analogous to these claims:
If you put your hand in the fire, you get burned. True.
If you get burned, you put your hand in the fire. False. Maybe you put your head in the fire. Or your hand on the stove. Or put your fire in the magma fist.
Okay, so what do I think the other possibility is?
(3) Logias qua Logias cannot be damaged without haki; whether or not they can be hit without haki depends on their element.
Logias precisely as logias, without reference to their specific element, or any fact other than that they are logias, do not take damage without haki infused attacks (or standard devil fruit weaknesses, or unfavorable matchups such as Enel vs. Luffy, none of which are obviously at play here and will thus be left out of the discussion).
But have we ever actually been explicitly told they can't be HIT without haki? It seems reasonable to think this from some of what we have seen, especially early on, but I think we have seen enough to actually contradict it. I want to claim that it has been specific elements doing the work:
Ace: Fire. Fire can neither be "hit" in any meaningful way, nor "damaged." Scare quotes signify that I am not even sure what it means to hit or damage fire.
Enel: Lightning. See Ace.
Kizaru: Light. I REALLY don't know what it means to hit or damage LIGHT.
Now, let's look at the exceptions…
Aokiji: Ice. Aokiji's element is solid. If someone punches Aokiji iced out, without haki, does his hand just pass through Aokiji? I think that's a bit ridiculous: Hands don't pass through ice. Furthermore, if Aokiji ices himself over, and someone punches him hard enough, I see no reason why that part of Aokiji shouldn't break/shatter. That is not to say that he would be damaged in any way - he could just reform. I think we may have even seen this at some point, but I'm not sure. The point is: Without haki, I would think you could actually HIT aokiji, though not damage him.
Caribou: Swamp. When he is hit, the attacker tends to just sink into him. This seems to entail that his element, in an importantly different way than lightning or fire, touches you. I would think that if Luffy, using no haki at all, bazooka'd Caribou, he'd splatter. Would he be harmed? I seriously doubt that - he could just reform, no problem. But can he actually be TOUCHED or HIT? Yes. (I think we've actually seen him carried without haki…)
(Interesting case: Caesar Clown: Gas. Can't be touched (there is an instance where Luffy grabs him with unblackened arms, and I DO think this confirms colorless haki; it just makes no sense, otherwise, to be able to grab a gas, unless this is an error that was missed by editors (see (1) above).)
I want to understand Trebol in a way similar to Caribou. Fire, Lightning, Light are meaningfully different than things like Ice, Swamp, or Mucus; the latter are solid/semi-solid, and as such can be touched in a way that lightning, for example, cannot.
So, even if Luffy DID use colorless haki, the point is this: Luffy hitting Trebol does not mean he was using colorless haki; since Trebol is a mucus man, you can touch him, even if you cannot actually hurt him. (Heck, he didn't even look banged up afterwards, but admittedly we only saw a couple small panels of his face after the hit.)
So, Luffy hitting Trebol does not confirm colorless Haki (even though I think other circumstances do, and though I accept colorless Haki).
This is not nearly as clear as it could be. I'll improve it if I can.
Anyway, it is possible I have said nothing interesting here. Maybe people already think these things. As a consolation prize, I think this is interesting: in relation to the "states" of logia elements - solid, gas, semi-solid… What about liquid? Specifically, what about the Water Water fruit? Is that even metaphysically possible in the one piece universe? I think we need to know more about devil fruits before being able to answer that. Would it be OP, since it would essentially have no weakness, or would it just result in an otherwise normal person who now can no longer swim and is constantly fatigued/passing out? I have no idea, but I think it would be a lot of fun to think about.