Green and purple go together with super-villains like nominative determinism. Just ask one verdant villain by the name of Victor von Doom, and his hordes of fascist followers. Or Annihilus -- the purple-green space bug that leads an undying hive literally just called The Annihilation Wave.
Then you've got your heroes. It's not hard to guess where a character like Captain America gets his red, white, and blue color scheme. Nor why Western comics tend towards the trio in general. Hint: It's because they're on all the flags, and comics were made in the wake of an era where everyone felt pretty good about colorful characters punching Nazis.
The concept has been in the back of artists' minds for decades. Although it's only more recently been addressed, if not outright explained, in the comics themselves. The most succinct example of which comes from Matt Fraction and Mike Allred's run on Future Foundation (or FF for short). Here the differences are beautifully broken down by colorist Laura Allred's typically stunning chroma.
The actual reasoning goes just a bit deeper than the children's matter-of-fact observation, however. Colors have a "language" to them that differs in various parts of the world. Red (the color of blood, muscles, exertion, and nervous first dates) typically denotes energy. Hence your quick and agile goodies like Iron Man, Daredevil, and Spider-Man monopolizing the hue.
Besides being printed in a very different time, comics were also produced quite cheaply. A tight spectrum of inks -- white, blue, yellow, green, and purple -- was leaned upon to cut costs. Heroes, who ostensibly stood in the light, got bright and straightforward primary colors. Meanwhile villains got more mixed, less direct tones to denote their more deceitful, shadowy personalities.
Sure, Marvel hasn't technically explained why all of its villains (and former villains) shop at the same tailor, but at least they've made it clear the mass coincidence hasn't gone unnoticed. Not to mention we got one helluva gorgeous splash page from the Allreds as compensation.
The Punisher fought in Vietnam. Tony Stark had his heart pierced by a shard of shrapnel in the Persian Gulf. The Fantastic Four first fought Galactus in 1966. Except they didn't -- none of these things happened, yet they're all true for the purposes of the Marvel Universe.
The concept of a "sliding timescale" has been around for as long as comics have needed to keep telling new stories with the same characters (and sometimes they same stories with new characters). The term refers to the very malleable, very convenient way in which heroes' and villains' back stories are altered to make them more relatable to a modern age.
These days, Frank Castle didn't serve three tours in Vietnam -- he was a Desert Storm vet.
Same goes for Tony Stark. Iron Man wasn't injured in the gulf (or even Vietnam, as was originally the case). He got his origins in Afghanistan. As for the Fantastic Four... Well, let's just try to avoid referencing specific years entirely from now on, shall well?
Actually, though, let's refocus on Galactus. The purple-pants behemoth has been a core facet to Marvel's more cosmic workings since, yes, the 1960s. More recently his long-time contacts on Earth -- the aforementioned FF -- are taking a sabbatical from reality. Leaving Galactus to interact with other agents of Marvel universal science and logic.
It didn't take long after Secret Wars, during which the Fantastic Four made their exit, for Galactus to start playing said major role again. The very first arc of Al Ewing and Kenneth Rocafort's reboot of The Ultimates sees the Devourer of Worlds become "The Lifebringer." Essentially, Galactus sets off to give back more than he took during his time eating planets.
What does that have to do with the Marvel timeline? Well, when he flipped his switch Galactus started moonlighting as an agent of the universe -- righting wrongs, and tracking down answers to big questions on behalf of reality. One such answer the big-guy-now-in-gold shares with The Ultimates is that time doesn't flow in a straight line. Major events, like superhero origins, get caught in the timestream's wake and are tugged along. "Always just a handful of years behind..."
And just like that several decades of head-scratching logic gets waved away as the universe "working as intended." If only it was so reassuring when customer support says the same thing in the real world.