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The Beihong agate from the southern section of the Xing’an Mountains in northeastern China is a kind of cryptocrystalline agate with a yellow to orange-red colour. However, it has been less studied in previous research, and there is a lack of quantitative study on the cause of its colour. In this study, the colour of Beihong agate was quantified by a colourimeter, and the quantitative relationships between colour and spectral characteristics of Beihong agate were studied by XRF, Raman spectra, UV-VIS spectra, and a heat treatment experiment. The results show a high correlation between the lightness and the hue angle of the Beihong agate. The change of total Fe content can significantly affect the lightness and the hue of the Beihong agate. The first derivative curve can effectively distinguish the relative contents of goethite and hematite in the Beihong agate, and the position of a primary trough is related significantly to the colour of the Beihong agate.