The undocumented spectral lines that helped identify radium were carmine in color.
In 1898, the Curies studied the radioactive residue left after uranium was removed from pitchblende. The residue behaved chemically much like barium, but its spectrum showed strong carmine-colored lines that had not appeared in the known spectra of barium compounds.
That combination was decisive: the material resembled barium chemically while displaying a different and distinctive emission spectrum. Spectroscopy therefore supplied evidence that the residue contained a previously unknown element, later named radium.
“Carmine” refers to a deep crimson-red hue, not violet, emerald, or azure. The wording can be confused with radium’s flame-test descriptions, which are also commonly given as carmine red or crimson red. In the discovery story, however, the key point is that the carmine spectral lines were new and helped distinguish radium from barium.