Art Nouveau is an international style of art of, architecture and applied art, especially the decorative arts, that was most popular betwen 1890 and 1910. A reaction to the academic art of the 19th century, it was inspired by natural forms and structures, particularly the curved lines of plants and flowers...
By 1910, Art Nouveau was already out of style. It was replaced as the dominant European architectural and decorative style first by Art Deco and then by Modernism. Source: Wilkepedia
"Midcentury modern" itself is a difficult term to define. It broadly describes architecture, furniture, and graphic design from the middle of the 20th century (roughly 1933 to 1965, though some would argue the period is specifically limited to 1947 to 1957).
“Modern” gets its name from the artistic movement of modernism that began with the end of World War I. Its 50 year span encompasses a shift from the ornamental “Hollywood glam” Art Deco designs of the 1920s to the streamlined organic forms that are characteristic of Mid-Century Modern in the 1950s to 60s.
Midcentury modern colors typically are greens, oranges, golds and browns.