Doing an Instrument Change within a piece makes the notes of the new instrument red.
I have a piece for violin and an addendum for cello. I wrote the cello piece separately and pasted it at the end of the violin part. (It is also in a different key.) Most of the notes in the cello part came out red, indicating they were out of the instrument's range, which they had not been in the original version of the cello part. It seemed the program thought I wanted the violin to play these low notes, as if I had not done the "Change Instrument" procedure.