My quick guess is that it is so dim, that our eyes are seeing it mostly with the rods (instead of the cones), which only see black and white. „In the night all cats are gray“

It depends on how bright it is where you are.

When it’s very very dim your color sensing part of your eyes, which are less sensitive to light, don’t work. Only the black and white parts of your vision work.

Kind of.

Uhhhh, I saw them yesterday in northen* hemisphere in Europe and they waren’t white at all… So my guess would be that it depends from where you look at it ?

But in reality they are more dull than on pictures, because photographers use Long Exposure to make the color brighter than they actually appear.

Edit: Typo

Because it’s travelling backwards in time. That’s why it appears in black and white. You’re only seeing the past version of it.

In my experience the aurora borealis is always green. I live in the north of Sweden.

I live too far south to see it but my understanding is that at different layers of the atmosphere, the stellar material interacts with different elements. So, it gets green or pink or whatever depending on how deep it goes.

It’s the green parts that look white / grey. I believe it’s more of an illusions - if you have something to contrast it with, such as the moon, you can start to see a slight green tint. The pink I saw last night was very noticeable though.