C₂4668 spectral index as a function of age. The yellow star is the measured value from the G140M spectrum of J-138717. The green, red, gray, blue, and purple lines are the predictions of the M05, M13, XSL, CvD18, and EMILES models, respectively. Dark to light colors indicate higher to lower metallicities. Both observations and models have been smoothed to the instrumental resolution of the BaSeL stellar library. The brown (olive) line in the central (right) panel indicates by how much the tracks would shift towards higher index values due to a C-enhancement(α-enhancement) of 0.15 dex (0.4 dex), as predicted by the CvD18 (MILES) models.