A bar’s staff lines will be colored red if the bar has at least one voice that’s not rhythmically stable. This is intended as a “heads up” to you, so you can fix the notation.
“Rhythmically stable” means the combined duration of all the notes/rests in the voice doesn’t match the time signature. You can fix this by adding notes, removing notes or changing existing notes’ rhythmic values.
This red coloring only happens during editing, not during viewing.
If your slice has a pick-up bar, in which the rhythmic width is less than the time signature, you can mark it as such with the “Mark as pickup bar” editor command. This will tell Soundslice not to color the staff lines red in that bar, because the note rhythms deliberately don’t fit the time signature.
Having a hard time figuring out exactly why your bar’s lines are red? Try rotating across all voices in the bar to make sure there isn’t a voice that you’ve unintentionally added a stray note or rest to.