This was the magic system in Changeling , by Roger Zelazny, and its sequel Madwand .
From the Wikipedia article: The strands he can use to perform magic are everywhere around him
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In Knights of Dark Renown by David Gemmell (1989), magic is tapped from "threads" of different colours accessible, with training, only by the "gifted".
The main character spies on a craftman-magician and eventually becomes his apprentice.
Green thread is nature-related and used for healing, black for Earth and for example used to make a sort of "boots of speed", and so on.
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I was googling the same terms looking for a specific book, which I eventually found!
The Homecoming Saga by Orson Scott Card.
I highly recommend this book.
Quote from Wikipedia: Hushidh Luet's older sister, she is a raveler: able to see the relationships between people manifested as threads in her vision.
This gives her remarkable insight into what people are thinking, as well as the ability to know what well-chosen words will break weak threads.
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That sounds like the magic system in the Chapel Hollow series by Nina Kiriki Hoffman.
The Thread That Binds the Bones is probably the one in question, it was published in 1993 and had a male protagonist.
He can shift his vision to see colored threads that he can manipulate and pull or weave to perform the equivalent of magic in the setting.
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Could this be Bad Magic by Stephan Zielinski (2004)?
Magic is real.
Strange forces and uncanny creatures abound.
Cosmic evil is loose in the world--especially around San Diego--along with transcendent evil, primordial evil, simmering evil, objective evil, and specific evil.
The main character created a cloak of invisibility by pulling threads of invisibility from the homeless.
(Not so much being invisible but, unnoticeable like people couldn't focus on you.)
Also, he would go to the hospital burn wards and pull out red threads of pain from the patients.
One to help ease their pain but
, He would also use them for offensive means.
I think he ran a shop and ended up getting pulled into some war between light and dark forces.
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I read this book between 1989 and 1998, I can't remember its title, book cover, or author. The only thing I can remember is a loose idea on how the magic system worked.  In this book the main character could see strands/threads of various lengths and colors in the environment. The character could pull on one of these threads to tap into the power. The color represented a specific element while the length represented the amount of power that could be pulled from the thread.
Fantasy book where character can see visible magic elemental threads of different colors and lengths
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<story-identification><magic>
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This may have been the magic system in Cageling or Madwand by Roger Zelazny; Knights of Dark Renown  by David Gemmell; The Homecoming Saga  by Orson Scott Card; the Chapel Hollow series by Nina Kiriki Hoffman; or Could this be  Bad Magic  by Stephan Zielinski. The book may be from the Chapel Hollow series by Kiriki Hoffman or is potentially one of the following: Changeling by Zelazny; Knights of Dark Renown by Gemmell; The Homecoming Saga by Scott Card; or Bad Magic by Zielinski.
The book is potentially one of the following: Changeling by Zelazny; Knights of Dark Renown by Gemmell; The Homecoming Saga by Scott Card; or Bad Magic by Zielinski. It may also be a book from the Chapel Hollow series by Kiriki Hoffman.