Assign a positive velocity to the red box and negative velocity to the blue box. Are they moving in opposite direction?

Yes, they are moving in opposite direction one to the other.

Explanation:

Velocity is a vector quantity, which means that it has both magnitude and direction. The magnitude shows the size of the velocity, and the direction shows which way it is moving in reference to a chosen reference direction. If the red box is assigned a positive velocity, and the blue box is assigned a negative velocity, as indicated in the question, then it means that the red box, and the blue box, both move in opposite direction to the other.

scientific laws are the result of testing theories and proving them to be facts.