HI David,
The thread Eoin links to will hold the answer, but to summarise:
G601-G603 (the initial batch, different cab window arrangement and bonnet length, no vacuum brakes) initially wore silver with red buffer beams and black(?) numerals. They later wore green and ended up in Black and Tan. Not sure if they wore the black and white livery, but @jhb171achill may be able to confirm.
The second batch of Gs were delivered in green but were repainted before entering service(!!!), so the livery on the green G at Downpatrick, while not quite fictional, was never observed in service. When they were retired, some wore black and white while others wore Black and Tan.
I should have research material at the office which should contain further info. I’ll have a look tomorrow if I have time.