Venom's offspring, the symbiote known as Carnage, is red because it bonded with its host Cletus Kasady's bloodstream on a cellular level.
When the Venom symbiote came to rescue its host, Eddie Brock (who is also known as the super villain Venom), from jail, it left an asexually-produced offspring behind. The spawn bonded to Cletus through a wound in his hand, merging with his blood and becoming red-and-black in color.
Venom #8 confirms symbiote colors matter, with red symbiotes being stronger than black. Red symbiotes signify a perfect bond with a host, potentially explaining their increased strength.
Carnage was red and stronger than Venom, Venom was also scared of Carnage because of his 'pure evil' ways and just being so violent. To note though Eddie and Venom bonded 'better' than that of Carnage with Cletus Kasady and Cletus was a serial killer if I remember rightly.
According to the comics, Carnage is Venom's (the Symbiote alien itself, not the typical venom form) son and evolved the ability to bond with his host's red blood cells, creating a stronger bond between symbiote and host. So in the comics, Carnage is red because he's soaked in the blood of his host.
Birth of Carnage
After this, Cletus bit Eddie's hand and instead of getting blood, the serial killer got an offspring of Venom, that would later be known as Carnage.
During a confrontation in a cathedral between the Venom and Carnage symbiotes, Mulligan was apparently killed by Barrison. However, he absorbs a piece of Carnage, which later revives him and causes his eyes to glow blue.
Since his debut however, Venom has evolved into an antiheroic figure, slowly distancing himself from his initial goal to ruin Spider-Man's life to try and do good instead, even putting aside his differences with and helping Spider-Man at times.
Sonic manipulator Shriek is commonly known as Spider-Man's foe, and as the sometimes-partner of Carnage. Born Frances Louise Barrison, it's believed that an unstable upbringing causes her frequent bouts of insanity. When paired with Cletus Kasady, her thirst for chaos is insatiable.
Riot is one of the five symbiotes forcibly spawned from Venom by the Life Foundation and is arguably the weakest of the group.
When a symbiote offspring of Eddie Brock, AKA Venom, merges with serial killer Cletus Kasady, the result is the nightmarish murdering menace known as Carnage! Reborn from an alien creature, he's a more psychotic and dangerous being that spawns a family of symbiotes.
The Blue Symbiote was one of the symbiotes from the planet Klyntar. Although it was able to bond with Jacob, it did not accept him and instead ate his organs.
Knull, the dark god of the symbiotes, is the creator of the first symbiote, Venom, and he waged a brutal war against the celestial beings, using his power to shroud the universe in darkness.
Venom also dislikes being addressed as a parasite, due to it being a vulgar insult on its homeworld. This is because Venom feels as though a symbiotic connection is meant to benefit both the host, and the symbiote, as they both power each other like a team.
Sometime later, Green Goblin abducted Peter Parker and injected him with the Venom symbiote. Green Goblin did this because he thought of his son, Harry Osborn, as a disappointment and thought that because Peter was intelligent he could be the son he never had.
As the first major, openly gay character created by Marvel Comics, Northstar generated significant publicity in the mainstream press, and Alpha Flight #106 sold out in a week, although the series was not a very popular title. It is the only comic book issue to have been inducted into the Gaylactic Hall of Fame.
Edward Charles Allan "Eddie" Brock is the human host of the symbiote Venom. Together, the pair are known as the Lethal Protector. After being transported to another universe as a result of a spell cast by Doctor Strange, he learned about the Avengers, Spider-Man and Thanos before being returned back to his universe.
In the Comics Carnage binds to Cletus blood which is a deeper bond and the fact that Carnage is Venoms child which each subsequent generation of symbiotes is stronger than their parents.. In The Venomverse, red is the strongest color of the symbiotes.
Venom (Eddie Brock) In Comics Powers, Villains, History | Marvel.
The new symbiote then bonded with Cletus Kasady through a cut on his hand, transforming him into Carnage. The bond between the Carnage symbiote and Kasady was not as strong as the bond between Brock and the Venom symbiote. However, Cletus Kasady was also a serial killer and thought of as insane.
That same issue also showcases his fully-formed symbiote form when he finally bonds with Officer Mulligan during a fight with his father and grandpappy. During this fight, one thing became abundantly clear: as strong as Venom and Carnage are, Toxin is much, much stronger.
Like a previous poster stated Venom feels responsible for Carnage and Carnage hates his “dad”. If you read the new Venom comics and the Web of Venom comics that set up Absolute Carnage they explore Eddie Brock's and Cletus Kassidy's teen/childhood.