Can I put any design or collection of art treatments in a crosswalk as long as the transverse white lines are present?
No. The FHWA has consistently stated since 1984 through eight Official Interpretations that nothing except an aesthetic treatment is allowed between the white transverse lines of a crosswalk. If non-retroreflective colored pavement, including bricks and other types of patterned surfaces, is used as a purely aesthetic treatment and is not intended to communicate a regulatory, warning, or guidance message to road users, the colored pavement is not considered to be a traffic control device, even if it is located between the lines of a crosswalk. Any street art should NOT distract from the crosswalk or make it less clear that path is a crosswalk because otherwise, accidents and liability may increase.
Additional guidance and a summary of past official interpretations on this topic is summarized here.
CLRP Traffic Signs and Pavement Markings Manual