I am thinking that it is equivalent to seeing the face of an animal from a faraway distance without any markings to indicate the mouth, like spotting a deer several meters away and only seeing their nose on the snout. It may also place emphasis on the expression of the eyes as well.

With Hobbes saying "It is like having a big white sheet of paper to draw on!" and the large empty space that follows, it seems implied that the reader could imagine -- and perhaps draw -- themselves riding alongside Calvin and Hobbes in their adventure. That gives it that mystical feeling for me.

I needed this in general, thank you

Blue seems cool.