In the Iron Fist TV show, the main character Danny Rand is the Iron Fist, the protector of the legendary city K'un L'un. As such, he can concentrate his energy in his fist, which then glows yellow and gives him extreme strength. Strong enough to blow a concrete wall in one blow or to hurt Luke Cage.
We can see a few other Iron Fists in Season 2 (spoiler ahead). The first one (let's call him A) is
Davos, who is Danny's childhood friend in K'un L'un. They competed for the title of Iron Fist.
His fist is red. I first understood it as the symbol that he is not a true Iron Fist, since he didn't beat the Mystical Dragon that traditionally grants the power of the Iron Fist but
stole the power from Danny, through a mystical ritual made by (surprisingly good at fighting!) New York tattoo artists...
The second one (let's call her B) is
Colleen, Danny's girlfriend and martial artist.
I believed her fist should also glow red since she got her power by a similar ritual than the character mentioned above. But her fist is white when she uses the power of the Iron Fist.
What explains the difference of colors of the Iron Fist?
My theories so far:
- A's power is corrupted, since he got it by an illegitimate method. Symbolized by a red fist. On the other hand, though B got her power by a similar mean, she has a pure white fist, maybe because the gain of her power was somewhat allowed by Danny. And also because she may be
the descendant of the first female Iron Fist.
- An Iron Fist's true color was always assumed to be white, but it is explained in Season 1 and in the Defenders show that Danny does not fully master his power and has doubts about his duty and place in the world. So yellow fist means uncomplete Iron Fist.
- Every Iron Fist has his/her own color, depending on his/her personality.
- Netflix's Marvel TV shows suck at explaining and keeping consistent their mystical elements. See all the questions about the Hand and Elektra on this site.
The show did not explain it so far, so I will accept Words of God, or answers from the comics.