Final answer:
The question appears to conflate artistic contour with barbershop terminology. In art, 'contour' defines the shape's edges, similar to how an outline shave gives hair a defined edge. A neck shave is a different grooming process that cleans up the neck area.
Step-by-step explanation:
The difference between a neck shave and an outline shave relates primarily to grooming and barbering, not the visual arts. However, since the question references an artistic example to describe contour lines, it may be beneficial to explain contours in the context of art and then clarify the barbershop terms.
In art, a contour is a line that defines the outline and surface ridges of an object. For example, the outline of the hat on one of the horsemen is clearly made by a few black contour lines. This visual technique highlights the edges and borders of shapes and is effective in creating a strong visual impression. On a white page, these black marks contribute to the perception of a three-dimensional object where there isn't one.
In contrast, a neck shave typically involves removing the hair from the back of the neck using a razor or clippers to give a clean finish to a haircut. An outline shave is about defining the edges of the hairline around the ears, neck, and temples. This provides a sharp, polished look and emphasizes the hairstyle's shape.