What is black when it's clean and white when it's dirty?

In 1907, the retrospective on the work of 19th century post-impressionist georges seurat influenced the development of cubism in a radical way through his painting titled, “sunday afternoon on the island of la grande jatte.” ? ?

The opening section sounds marchlike, with accented beats and alternations between instrumental groups. eventually, a total change of character marks a short, new section. at approximately which time interval from the beginning does this appear?

Joke-- what is the best way to save money?