Color photography and color printing did not become practical until the late 1930s. Photochromes, or chromes, are prints made from color photography. In the postcard world, "chrome" usually means a full-color card printed on highly glossy stock. Photochrome postcards in this sense first appeared in 1939, and took off in popularity after the end of World War II. This is the most common style of postcard still seen as souvenirs.
This photographic postcard depicting pre-game football practice at the "Natural Bowl" at St. John's University, from about 1974, is an example of a chrome. It may have been part of a postcard booklet; it has perforation marks along the left edge.
This College of Saint Benedict Blazers postcard, with symbols of many intercollegiate sports, is also an example of a chrome, circa 2003.