How can I judge when landscape colors are overdone, especially with a green-tinted ND filter?

Hello Rebel and welcome!

Color is never overdone and color is always overdone. See? It's that simple. :- )

Your photograph is what you want it to be, what you want to convey to your viewers, what emotional response do you want? Your laptop is your darkroom. Make what you want.

Of course it is easy to go overboard. The internet is full of oversaturated photos which collect a great amount of likes but, in the end, are unsatisfying.

@Caleb had a great idea. If you want to reproduce accurate colors, shoot a color chart and correct to that. But, you might not want accurate colors, you might want to evoke something different.

I too have shot with heavy ND filters. This is, if my memory serves, a 4 minutes exposure.

It was probably shot with stacked ND filters. So how do I know what the scene looked like? I shot it with my phone also!

As you can see, the colors are similar, but not identical. Now lets visit the same place at a different time with different light.

And here's what my camera saw.

And yeah, it was an older iPhone so the quality is really awful on that. But notice how the original was drab, but the one with ND filters was able to see the green that my mind say, but that the scene only slightly contained.

And finally, don't be afraid to go to B&W, that is a completely legitimate form of expression. I shoot both and you should consider it also.

And you know what is cool about that? I believe the first, brownish, image and the last B&W one were shot at the same photo shoot. You, the photographer, create the scene.