Throughout the pages of Marvel Comics, the character of Drax the Destroyer is depicted as a green being with red markings all over his body.
When a fan brought up the color of Drax, Gunn said that it was really just about the landscape of the rest of the MCU.
Gunn continued on in the thread to answer another question about Gamora, saying that there were plans to make her blue but they decided against that due to the appearance of her character in Avatar.
The green and red design for Drax may have brought up some serious Christmas vibes, which could have been distracting throughout the various Guardians and Avengers movies.