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.