Electric cars are all green, gas cars are green if they meet EURO 1, and diesel cars are green if they meet EURO 4 (or EURO 3 + retrofitted particulate filter)[1]. This is pretty loose for passenger cars: any made in the last fifteen years should all qualify [2].

[1] https://www.germanemissionssticker.com

[2] https://en.wikipedia.org/wiki/European_emission_standards