Answer:
As to how chromium was discovered, that may be lost in history. Some of the tips of bronze spears of the Terra cotta Army, ~300 BCE, show no corrosion, and have been found to be coated with a chromium compound. That has to be a long-lasting coating!
Chromium is so named because many of its compounds are brightly coloured. Chroma = colour in Greek.
Some chromium salts are quite toxic, and are part of the story line of the film Erin Brokovich. This concerns the hexavalent form.
In modern times, it was first identified by Voiquelin around 1797. Probably in the analysis of a bright red lead chromate.