It is unlawful for any person to paint, letter or color any street, sidewalk or curb and gutter for advertising purposes, or to paint or color any street, sidewalk or curb and gutter for any purpose, except as the same may be done by city employees acting within the course or scope of their employment; provided, however, that, this provision shall not apply to uniformly coloring concrete or other surfacing, or uniformly painted house numbers, as the coloring may be approved by the city.