You can change the color of notes in the Piano Roll Editor to display note velocity, region colors, or MIDI channel colors. By default, notes correspond to the region color.
Do one of the following:
Choose View > Set Note Color in the Piano Roll Editor menu bar, then choose an option from the submenu.
Velocity Colors: Colors notes to show their MIDI note velocity.
Region Colors: Colors notes to correspond with the MIDI region.
MIDI Channel Colors: Colors notes to correspond with the MIDI channel.
Control-click the Piano Roll Editor background, then choose a color assignment from the shortcut menu.