What do the various lights in the overhead aisle light cluster of a Bombardier Q400 NextGen represent?
This is the so called &quot;area call panel&quot; and it is indeed used to signal the cabin crew. The colours and their respective meaning is not regulated and depends on the aircraft and aircraft manufacturer.
A selection of colors, manufacturer and type according to airliners:

Boeing / McDonnell Douglas:

Pink - Flight crew to FA call ORFA to FA call
Blue - Call to FA from passenger in seat
Amber - Call to FA from passenger in lavatory
Red (717 only) - Lavatory smoke detector set off
Green (DC-9 only) - Unused, included as a placeholder for an unknown function.

Airbus:

Red - Flight crew to FA call OR (some aircraft only) FA to FA
call
Blue - Call to FA from passenger in seat
Amber - Call to FA from passenger in lavatory
Flashing Amber - Lavatory smoke detector set off
Green (some aircraft only) - FA to FA call


Further down you can read, that Bombardier Dash 8 uses solid green for a regular call and flashing amber for emergency call.
A definitive answer can only be found in the respective aircraft or crew manual.