The artist is choosing to emphasize the boy in black.  The artist has made him the focus of attention by painting him with dark colors and by having the other figures gaze at him.  The use of bright light is also drawing the viewer's attention to this figure.

The main emphasis of this work of art is the yellow rectangle.  The viewer's eye is drawn to this shape through the placement of curved and wavy lines that make one's eye move towards the rectangle.