The Chrome Era is generally considered to be from 1939 to the present.
Here’s an informative summary from Postcards of Hamilton
“Today we think of chromes as any modern, glossy surfaced card. The term “chrome” originated with Kodak’s Kodachrome film. This colour-reversal film that could be used to produce positive images was first introduced in 1935. It was not long before photographers and printing firms (including postcard manufacturers) began to use colour positives. Chromes were the first colour postcards produced from colour film and printed with the photo-mechanical halftone process. The “chrome era” begins with the publication of a series of 85 chrome postcards by the Union Oil Company of California in 1939. The cards originated in the San Francisco office and were sent out to dealers to be given out free at their gas stations. Large national postcard publisher/ printers such as Mike Roberts, Plastichrome, Dexter Press, Curt Teich and others followed in the production of modern chromes. In the beginning, the traditional 3 ½” x 5 ½” size of postcard was adhered to but by the 1960s the continental size of approximately 4″ x 6″ was becoming more popular. “