So, we all know the egg is red, I mean, that's kind of hard to miss.
But something I want to bring up, is how red is a primary color.
Although that's useless information, until we remember what other color that's been used in the Dream SMP before, and another that was just alluded to in Tales From The SMP are primary colors too.
Blue is easy to remember, it's Ghostbur's signature thing! He wanted to give out blue to soak up sadness so everyone could be happy!
And yellow was alluded to yesterday (as of writing on January 25, 2021) in Karl's most recent tale "The Lost City of Mizu" where in Skeppy's room Ranbob (The "tour guide") said he was blue, but was known to turn red and/or yellow. (It should be noted that while a lot of the information in the city was untrue, or blatant twistings of real events/people in the present day SMP, there's always a ring of truth even in it.)
So what does this mean? Well, they're all connected, in a way. But how? My personal theory is that they're all tied to emotions/feelings/ideas in a roundabout way.
Red is chaos, anger, violence, it's fiery and passionate, and can also represent love.
The egg is spreading over the SMP, to take it over. But most notably is how Bad and others being controlled talk about the egg. It's almost an admiration, a love, for it. They want to spread the eggs influence, and in a certain way, spread its love.
Of course though, there's how the egg is taking over the SMP. It's not a peaceful takeover, sure it's slow and steady and if you blink you could miss it, but that's just the spread, not what Bad and Ant are doing. They're doing whatever they can, including trapping people and nearly killing them, to make them go under its influence.
It may not be violence in the sense of total bloodshed or going the Techno route and blowing things up but again, it still isn't peaceful.
Blue is softer, it's sadness and pain, but sometimes it can be a healing color like water.
Blue never had anything physical to grow like the egg and its vines. But, it still had someone to grow its influence. Ghostbur, who wanted to help people, who wanted to make sure no one could be sad. Everywhere he went he always offered people blue if they needed it. Something that started off clear, with no emotion, and became blue by soaking up sadness.
Ghostbur is gone now, whatever his unfinished business was (Whether it was the defeat of Dream, or Tommy and Tubbo getting their happy ending, or even just the return of L'Manburg's values to the two they mattered most in) is done. So there's no blue left to be given out by him. There's no vessel for the color to spread itself with, unlike the egg.
And then Yellow. Happiness, joy, warmth, hope. It's a very bright color, with very good meanings.
So this one obviously doesn't have a "vessel" to spread itself with. Except, if we take that meaning of hope, it does. It spreads through everyone, through harmony and community.
But obviously, it's going to be physical if Skeppy may turn yellow in the future.
In my opinion, it's also is what's going to save the SMP from the egg. Maybe Karl finds something in the past and brings it back, or maybe because Tommy is immune we learn that it isn't plot armor, it's because he has hope. Tommy is a fighter at heart, and that hope to win, even when overshadowed by unlikely odds, still exists.
Even Puffy shows this well. She's seemingly immune to the egg too (despite not being immune before, but I hold what was happening before as a filler plotline while the other plots were tying up, it was just to show what WAS going to happen later), realizing what it and the Badlands are doing is wrong. She wants to help people, save them, be a knight.
Of course though, these are only my thoughts and feelings on the matter and me rambling about what I think the breadcrumbs lead to. I do think though, that a message of hope and togetherness is what can help defeat a monster is a nicer message than the egg just taking over the SMP and it winning. (which I doubt is the angle that will be used.)