It's a Speculation
King wears a black suit but we all wanna know how he actually looks like? Let's see what we can make it out with the information we have so far
Skin Complexion
The panel below kind of give the impression that he has dark complexion with light colored hairEmotional Attachment
In panel below, King became furious when zoro cut his mask. He almost went bersekThere is one emotion Oda has clearly highlighted here - Anger.
Connection with sanji
There are few symbolism between Sanji and King.
1)Both wear Black outfits
2)Both can ignite fire though with different degrees
3) King can fly whereas Sanji has sky walk/moon walk
4) King belongs to the race Lunarian and queen bringing this up in front of Sanji.
5) Similar Hair color
6) Sanji has shown being on fire whenever he gets Angry (emotional trait shown by King when zoro cut his mask). It's kind of like that Anger enhances Sanji's strength.
Sanji vs Jabra
Sanji was angry and unleashed DJ. He also implied that gaining more heat/fire means more speed - In chemistry particles of gas gain more Kinetic energy with increase in temperature.
Sanji being pissed about his TS. He used Grill shot ( fire realated) here and usoop hyped his speed.
Angry Sanji shown turning into a monster with fire all over him after remembering his TS ordeal and used Hell Memories
Angry Sanji portrayal in Thriller Bark
Sanji dodging kata bullets and look at his glare at kata (he is pissed)
And, most recently Sanji being angry and unleashing Ifrit Jambe
Sanji was angry and unleashed DJ. He also implied that gaining more heat/fire means more speed - In chemistry particles of gas gain more Kinetic energy with increase in temperature.
Sanji being pissed about his TS. He used Grill shot ( fire realated) here and usoop hyped his speed.
Angry Sanji shown turning into a monster with fire all over him after remembering his TS ordeal and used Hell Memories
Angry Sanji portrayal in Thriller Bark
Sanji dodging kata bullets and look at his glare at kata (he is pissed)
And, most recently Sanji being angry and unleashing Ifrit Jambe
7) Do you guys remember that Judge made Sanji wear an Iron mask and locked him in dungeon and held his funeral? It was to erase the identity of Sanji. And, King also wear a mask.
Inferences and Speculations Drawn
1) King's Mask - He is wearing mask to hide an aspect of his identity. Perhaps his face or whole body is burned. My speculation is that during early childhood days King because of his own fire got his face/body burned as he didn't have strong body to protect himself from it.
Remember that sanji himself said that because of exoskeleton now he can have stronger fire.
Thus, king was mocked and humiliated and he wore mask to hide his face or body or he was forced to wear it.
That dark complexion is nothing but his burned face/body.
2) Stronger King - Whenever his mask would be cut open, king will become furious and with his anger overwhelming him and his fire will become stronger. So, if I am right then King vs Zoro is far from over.
3) Sanji most likely is Lunarian. Maybe, Sora was Lunarian or has some connection. Too much going on to ignore this even when Queen remarked that sanji can't be Lunarian.
4)Sanji's Anger emotion has direct bearance on his will power and physical abilities especially fire and speed. Also, as his fire will become stronger, his speed will increase in proportion to it.