When Venom said, “That’s a red one”, he was referring to Carnage, who is a result of one of his offsprings with serial killer, Cletus Kasady.
For Spiderman comic book readers, understanding the context in which Venom referred to Carnage as, “[the] red one” is easy, as the comics share the origin of Carnage. However, for film aficionados of the Marvel character’s world, it is less known. Therefore, in needing to understand when Venom would refer to Carnage as “a red one”, it is important to look at the origin of the character and his relation to Venom.
The comic books do a better job of telling the origins story. This is as Carnage’s inception was due to one of Venom’s offsprings bonding with serial killer, Cletus Kasady. Carnage’s inception happened after Venom, who shared a cell with Eddire Brock, broke through the prison where they were held. During their escape, a small piece of symbiote was left behind. Kasady came into contact with the symbiote, which merged with him when he was placed in the cell which previously held Venom and Brock.
As a result, Carnage was born. Moreover, his red colour has been chosen for more than just aesthetics. The colour is a strategic choice, which is continually attempting to further represent the attributes of the character. The colour red was chosen specifically for the character as Carnage’s inception happens through blood.
Additionally, the character’s dysmorphic and fluid body is meant to emphasise that Carnage’s inception happened on a molecular level. Hence, even his form is also unstable, much like Kasady’s mind. A fun fact is that Carnage is more powerful than Venom.