Who can move first in this situation?

You were right: you may move first.

In this situation you are driving on the main road, while the red car is coming from a side road/field path on the right and wants to enter the main road. In Germany, traffic entering from a subordinate road must yield to all traffic on the priority road. So the red car has to wait until the road is clear.

The blue tractor is on the same road as you, coming toward you in the opposite direction. That does not automatically give it priority over you. Since you are not turning across its path, you can continue first and the tractor simply continues in its lane as well.

Rule of thumb for similar questions: vehicles coming out of side roads, driveways, or field roads must always wait for traffic already on the through road; oncoming traffic only has priority over you when your maneuver would cut across their path (for example when turning left).