Here's some links to educate yourself:

K y'all... I have a bit of speculation on the green blood and i don't think its going to be brought up again, unless in Marcy's journal or in an interview, but like why not have fun theorizing?

And so here we are :)

*Looking back at this, it's not really all that insightful but worth a read if you are intrigued by the biology behind green blood

The Deliberate Change from a BLUE tank to a GREEN tank

So.... if you recall the whole 'True Colours' airing trainwreck, you might recall how the intro to S3 was released as a post-credits scene

In the version that was released Marcy was suspended in a blue rejuvenation tank. However, there was a deliberate change of this to a green rejuvenation tank.

So obviously... there has to be some sort of significance attributed to the green colour. And why can't it relate to Marcy's blood?

I wasn't in the fandom at the time, so I'm not at all sure what the theories about this were about. The most obvious is that it was a result of her calamity stone... perhaps that the tank was making use of the power of the green/wit calamity stone to sustain her life? Or that it was using the stone to 'weaken her' in her 'humility' to help w/ the possession? Or that perhaps that there was something within the tank/related to the technology of the tank that was manipulating with or changing the chemistry of Marcy's blood cells in order to help keep her alive?

Nonetheless... I have an alternate idea regarding this based on a bit of quick research regarding 'green blood'.

2. On a Medical/Biological Perspective

Basically, I found this very intriguing article titled, "These Lizards Are Full of Green Blood that Should Kill Them"

Here are some pre interesting snippets of info that are quite relevant:

"Red blood cells only live for four months or so, and when they die, our bodies recycle the iron within them. That process inadvertently creates a green pigment called biliverdin, which is then converted into a yellow one called bilirubin. Both of these are toxic, and our livers quickly filter them out of our blood. When you get a bruise, the unsightly green and yellow colors come from the biliverdin and bilirubin unleashed by dying red blood cells."

So it's normal to have green blood because of dying RBCs? Didn't know that. This validates the more prominent theory that Disney didn't allow for Amphibia to literally showcase red blood.

Also check this out:

"human blood is green in the deep ocean"

"Since the green reflectance peak of blood is always there, blood can be obviously green anytime you have a light source with no red color, and not just in the deep ocean."

AND THIS: "Why some animals have blue, green, or purple blood":

"[Horeshoe crabs]' blood uses a different protein, called hemocyanin, to bind oxygen. Because that binding process involves an atom of copper, rather than iron, the blood has a blue appearance when it's oxygenated, and little or no color when it's not."

"there are other marine worms called polychaetes that have blood with yet another oxygen-binding protein, called chlorocruonin. Chemically, it's similar to hemoglobin, and uses iron to bind oxygen. And though it looks red when concentrated, when it's more diluted, it can appear bright green"

Also this:

" "It's so rare [(referring to a condition called sulfhemoglobin)] that we don't have a perfect understanding how it happens, but some drug donates a sulphur group that binds to the hemoglobin molecule and prevents it from binding to oxygen," Flexman explains. "And that gives it the green colour." "

"According to Star Trek lore, however, Vulcans have green blood because the oxidizing agent in their blood is copper, and not iron."

And here's more on the green-blooded lizards, referenced prior:

"They should be dead. Biliverdin can damage DNA, kill cells, and destroy neurons. And yet, the lizards have the highest levels of biliverdin ever seen in an animal. Their blood contains up to 20 times more of it than the highest concentration ever recorded in a human—an amount that proved to be fatal."

" “It’s an unusual physiological trait that’s only found in New Guinean lizards, so it probably evolved once,” says Austin. "

"The team can’t rule out the possibility that the lizards evolved green blood once and then reverted to red on several occasions, or that there’s an even more complicated history of color-switching. But whatever the case, the pattern is more intricate than he suspected. At the very least, it means the lizards need to be reclassified."

^^I think this idea of blood could really correlate to the idea of regression and evolution within the species of Amphibia. And it seems that members of the Leviathan empire were only able to sustain themselves through medical developments and relying on biomechatronic body parts, before they met the shadowfish and created the core. It's kind of like they chose this path of 'evolution' for themselves and yet, in many ways, as a result of their greed and corruption, devolved. lol but im really just here going out on a tangent this point in particular is not really all that insightful...

CONCLUSIONS:

Exhibit A:

You can see the emblem depicted on the chair, which very much resembles Darcy's eyes and as a result a lotus flower

Exhibit B:

Same thing but on the wall (This is from the flashback in The Core and The King)

Exhibit C:

The new emblem, depicting the crown Andrias wears. It seems to represent the control that the Core has on Andrias, in turn implying the stronghold and dominance the Leviathan empire/the Core continues to have on the Amphibian way of life. That Amphibia, as a result, resists change and neglects to move on and adapt. That Amphibia, in turn, is succumbing to the monster depicted in the prophecy: corruption and greed

not sure if this is just for the sake of 'vibes' or some way to correlate the technology to having been created by studying 'shadowfish' but i do think its quite intriguing

also wondering if this is a possible connection to the strange shadowfish-like monsters in the moon on the promo

For starters, there is an evident change we see in Marcy throughout Amphibia, however it isn't of the like we see in Anne or Sasha

In Marcy we see someone who neglected to change, or moreso neglected to accept and adapt to the change as it occurred within her life

Marcy and her relationship with Anne and Sasha, as we see depicted in flashbacks throughout Amphibia, seems to have already been in shambles prior to them going to Amphibia. In Reunion we see how Sasha and Anne would regularly hang out w/out Marcy. And the reason, as we make it out to be in Marcy at the Gates and True Colors, is that her obsessions caused her to be negligent and oblivious to the world around her. She neglected to actively participate in her relationships w/ Sasha and Anne, that it ended up distancing them from eachother.

It was only during True Colours when she truly was confronted w/ the repercussions of her actions. Of course, we aren't completely aware of whether or not she reflected on her actions while she was in Amphibia, and what kind of personal development she went thru. The reason is that she always appears to sport some sort of a facade as she goes on role-playing her way to the top in Amphibia. It's as if she was 'faking' having gone thru change. And it's what makes her 'betrayal' to Anne and Sasha all the more horrifying.

The reason she was interested in 'travelling' to another world w/ her friends instead of leaving them behind is because she was already so alone. She didn't want to lose the sole relationship she'd ever found happiness in. And I think there's definitely the idea of Marcy making their relationship out to be more than it ever was, out of the fear of exploring the new.