To do that you will go to:
  1. Core
  2. Communication
  3. Resource Boards
  4. Navigate to the Persona Board you want to change
  5. Click on the Board that you want to add the color to
  6. Click the Paintbrush Icon on the left 
  7. Select the Segment you want to add the color to and select the color to add 
  8. Click Apply Changes
If you want to make changes to multiple segments, you would select each and apply the color and the click Apply Changes