How did colored lines in early music manuscripts help establish pitch?

In early music manuscripts, colored lines were a crucial innovation that helped establish precise pitch by creating a reliable visual grid for neumes.1

Before this development, singers relied on neumes—graphic signs that only provided an approximate indication of the voice's rise and fall—to help them remember melodies they already knew by heart. To move toward a system that could be sight-read, scribes began using horizontal lines as a reference framework.1

The use of color specifically served to anchor the system to fixed pitches:
* Red lines were typically used to signify the pitch F.21
* Yellow lines were often used to signify the pitch C.21

By marking these specific lines with color, or by placing the letters F or C at the beginning of the lines, musicians could identify exactly where the semitones of the scale occurred (specifically the E–F or B–C intervals). This provided a definitive reference point for all other notes placed on the lines or in the spaces between them, effectively transforming musical notation from a simple memory aid into a precise system for representing pitch.12