It may be helpful to quote the whole paragraph which contains the OPs query, for context.
She did not reply at once and he glanced at her sharply, accepting the pain it gave him. She was so lovely. Queen Nefertiti in a Dior ensemble. Her clothes seemed a part of her. Her plum-coloured redingote with its absurd collar arched like a sail emphasised her slenderness. Since it was fashionable to do so, she looked bendable, bone and muscle fluid like a cat’s. A swathe of flax-white hair protruded from a twist of felt, and underneath was something not quite true. Exquisite bone hid under delicate faintly painted flesh, each tone subtly emphasising and leading up to the wide eyes, lighter than Scandinavian blue and deeper than Saxon grey. She had a short fine nose and a wide softly painted mouth, quite unreal, one might have thought, until she spoke. She had a husky voice, also fashionable, but her intonation was alive and ingenuous. Even before one heard the words one realised, albeit with surprise, that she was both honest and not very old.
This, then, is a descriptive paragraph concerning one of the characters.
In context, we can intuit that the "twist of felt" is part of that character's clothing, which has been described in some detail. In particular, it's the
plum-coloured redingote with its absurd collar
The term redingote is unfamiliar, but according to the Merriam-Webster Dictionary it can denote several different items of clothing, the most relevant of which is:
a woman's lightweight coat open at the front
The "twist" is thus the collar of this garment, a light coat made of felt, which will be folded over to create the collar. We can intuit it is the collar by what comes next, which is clearly a description of the gaze following up the woman's neck to her "wide eyes". By extension, the "swathe of flax-white hair" must therefore be the woman's hair, tucked into the collar of the coat.
"Each tone" refers not to the skin colour but that of her make-up. We know this not only because someone's skin tone does not tend to vary, but because it's directly after the reference to "delicate faintly painted flesh" which is suggestive of carefully applied cosmetics.