Good question! I think it depends on context. If you're referencing the Pop Art movement (historical period), it is typically capitalized. All of your examples are referencing it as a movement. It's also suitable to write it as "Pop art" in this context: "I went to see the Pop art exhibition." However, if you're not referencing it as an art movement, you can keep the letters lowercase: "His room is covered in pop art posters." Generally, unless an artistic movement took place during a specific time (Baroque, Renaissance, etc.), you can choose to capitalize or not capitalize, depending on your desired meaning and context. I looked into this a little further, and it's actually quite controversial! See this link, starting at "Art movements, periods, and styles:" http://www.artedit.org/styleguide.htm

It is the name of a fairly well-defined type of art. Since there is just one type of this art and it has become well-known and accepted, it is considered a proper name. However, you wouldn't say Water Color Art, because it just describes the medium. It doesn't say anything about the style, subject or purpose - just that the art is made from water colors.

I would smile except that for whatever reason, I don't smile that often. Look through my profile you'll see why write that I would smile.