Why were cesium and rubidium named for colors in their spectra?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — alkali metal Modern version of the periodic table of the elements (printable).

Cesium and rubidium were named for the distinct colors observed in their emission spectra during their discovery by German scientists Robert Bunsen and Gustav Kirchhoff in the 1860s.123

Cesium: The scientists identified the element by two unique blue lines in its spectrum. They named it after the Latin word caesius, which refers to the blue of the sky.23
Rubidium: While analyzing the mineral lepidolite, the researchers observed two prominent red spectral lines. They derived the name from the Latin rubidus, a term used to describe the darkest red color.13

These discoveries marked the first time elements were identified using spectroscopy, a method that remains vital for identifying elements based on the characteristic light they emit when excited.243