Spoilers for Iron Fist Season 2 ahead! Danny Rand's training brother Davos on Iron Fist has always been jealous, and rightfully so, that an outsider got the destiny he was raised to believe belonged to him. In Season 2 of the Netflix series, Davos takes his jealous into his own hand — pun totally intended. Why is Davos' fist red in Iron Fist? When he takes the power of the Immortal Iron Fist back from Danny Rand, it looks ever so slightly different.
After collecting a special and one of a kind bowl, some cool tattoo chicks with steel needles and Danny himself, Davos is able to perform a ceremony that transfers the glowy-hand powers. Thus, he becomes the Steel Serpent. But why is the fist red this time? What's different about it?
Here's an educated guess. You know how in the Marvel Cinematic Universe, Steve Rogers had to be the perfect All American good boy so that the serum would react the way the Strategic Science Reserve wanted it to and turn him into a good boy hero? Then, when both Bruce Banner and some Hydra scientists tried to recreate the super soldier serum, it turned Banner into the Hulk, Emil Blonksy into the Abomination, and Johann Schmidt into the Red Skull? (The serum was also used on Bucky Barnes, but that's a whole other story.)
The point is, just like the super soldier serum or its imitations give different humans different super powers that reflect who they are inside, it seems like the Immortal Iron Fist takes on different looks depending on who wields it. This would seemingly also explain why he bears a serpent on his back instead of a dragon, etc. Later on, when Colleen wields the fist, it glows white — like grandfather's katana and her costume in the comics.
In the comics, Steel Serpent has more of purple look — but Jessica Jones' villain has the market on purple cornered. It makes sense that the Netflix series went in a different direction. Davos is sometimes known as the Steel Phoenix, too, and those birds are pretty red. His attack move is called the "Phoenix Blow" — so the imagery makes sense.
However, there is an issue of a team-up between Iron Fist and Spider-Man that features Davos taking Danny's power and his fist glowing red on the cover. In that issue, the power of the Iron Fist turns against Davos and almost consumes him. Maybe the red glow is actually a bad thing.
Davos' isn't necessarily evil. Heck, until he started "purging" the city by way of murder, you may even have agreed that he deserved to be the Iron Fist. He certainly would never have abandoned K'un-Lun or allowed it to be destroyed. That said, is the fist really all its super-powered knuckles are cracked up to be?
When Danny really thinks about what he's fighting for, he isn't sure beyond the fist itself. If pressed, Davos may not know what he's fighting for either. The fact that this power, the Immortal Iron Fist, takes on different colors when wielded by different people says that its more about the individual than whatever it represents. Or, like, it's just red because Davos is lowkey a villain on Iron Fist? Metaphor or not, it at least sets the two brothers apart, which might be the writers decided to do it in the first place.