The red is from the red thread showing through, sort of a translucent look. On classical (nylon) strings, the first three are plain clear nylon (usually) and the last three (bass) are wrapped with a soft metal thread, usually bronze.

I'll be experimenting with some other styles shortly, maybe using the wrapped strings for a sort of Copper-Johnish type fly.

Quote Originally Posted by Uncle Jesse View Post
Gig, Is the red from the thread underneath the string or did the string have some color to it. I am completely non-musical so I have never looked at a guitar string for more than 10 seconds.