Why should you brake now?

You should brake because of the green car: it is the vehicle that can most directly become an immediate hazard for you, so you need to be ready to reduce speed and increase your safety margin.

Rule of thumb in German driving theory: you brake early whenever another road user might enter your lane, stop unexpectedly, or force you to give up space, especially when visibility and grip are poor. On a snowy road your stopping distance becomes much longer and steering corrections are harder, so you must create a bigger time gap and avoid “late braking”.

Even if you are on a priority road, right of way does not protect you from a collision. You still have the duty to drive in a way that you can react to foreseeable mistakes by others. That’s why you brake (or at least clearly reduce speed) when the green car’s behavior could require you to slow down or stop.

The blue car and the tractor are not the key reason here because, in this situation, they are either already moving predictably ahead of you or are less likely to create an immediate conflict than the green car.