The names of rubidium and caesium come from the most prominent lines in their emission spectra: a bright red line for rubidium (from the Latin word rubidus, meaning dark red or bright red), and a sky-blue line for caesium (derived from the Latin word caesius, meaning sky-blue).
ðə nˈeɪmz ʌv ɹuːbˈɪdiəm ænd sˈiːsiəm kˈʌm fɹʌmðə mˈoʊst pɹˈɑːmɪnənt lˈaɪnz ɪn ðɛɹ ɪmˈɪʃən spˈɛktɹə: ɐ bɹˈaɪt ɹˈɛd lˈaɪn fɔːɹ ɹuːbˈɪdiəm fɹʌmðə lˈætɪn wˈɜːd ɹˈuːbɪdəs, mˈiːnɪŋ dˈɑːɹk ɹˈɛd ɔːɹ bɹˈaɪt ɹˈɛd, ænd ɐ skˈaɪblˈuː lˈaɪn fɔːɹ sˈiːsiəm dᵻɹˈaɪvd fɹʌmðə lˈætɪn wˈɜːd sˈiːsɪəs, mˈiːnɪŋ skˈaɪblˈuː.