What was the first time that we know that non-human animals have color vision? From what I can find, the first person who scientifically came to that conclusion was John Lubbock in 1888. What was so strange about it was that he studied Daphnia - a plankton (!). He discovered that the plankton was attracted to yellow light, but not to white light. Lubbock also reasoned that insects must have color vision, but was not able to scientifically demonstrate it. It was decades after Lubbock published his book, in 1914, that Karl von Frisch established the behaviour training method for testing color vision. Very soon after, German scientists applied the von Frisch method to every animal that they could get their hands on.
Apparently, before the late 19th century, it was simply assumed that animals don’t have color vision.