Short for chromolithograph; a brightly colored picture or print made using the chromolithography process, popular in the late 19th and early 20th centuries.
From 'chromolithograph' shortened through common usage; 'chromo' became popular slang for these colorful prints.
Chromos were the mass-produced posters and trading cards of the Victorian era—bright, affordable, and collectible, they essentially invented popular visual culture before photography became cheap.
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