Lights on Privately Owned Vehicles: “Section 7E. No motor vehicle operated pursuant to section seven other than fire apparatus, ambulances, school buses, vehicles specified in section seven D used for transporting school children, and vehicles specified in section seven I shall mount or display a flashing, rotating or oscillating red light in any direction, except as herein provided; provided, however, that nothing in this section shall prohibit an official police vehicle from displaying a flashing, rotating or oscillating red light in the opposite direction in which the vehicle is proceeding or prohibit fire apparatus from displaying a flashing, rotating or oscillating blue light in the opposite direction in which the vehicle is proceeding.” – MGL 90-7E
https://www.mass.gov/files/documents/2016/09/ws/125-evo-awareness-student-guide.pdf
I am a Uber driver and got pulled over for having a green uber light on in Merrimac. Police where fine but explained the law and told me not to have it on unless in am pulled over looking or waiting for someone. So, uber and Lyft drivers beware.