asymptotic giant branch (AGB) [@marigo08]. We sampled 10$^7$ M$_\odot$ of stars from a @kroupa01 initial mass function and then obtained effective temperature, gravities, and the F606W, F814W, and K-band magnitudes for each star from the isochrones at ages from 4 Myr to 14 Gyr. To obtain estimates for the CO line strength, we used the relations from @doyon94, who fit CO line strength as a function of effective temperature for dwarfs, giants, and supergiants. The total CO line strength at each age was calculated as the mean CO line strength weighted by the K-band luminosity of each star. We note that the broader @doyon94 CO index has values that are 20-50% lower than the one we measure here, nonetheless, the trends of CO depth with age should be robust. We also caution that the models remain very uncertain, especially given that the CO strength can be dominated by the coolest, thermally pulsing AGB stars, which are very difficult to model [@marigo08]. The CO line strength shows peaks at two ages; first at 10-20 Myr due to the presence of bright red supergiant stars, and second at $\sim$300 Myr from AGB stars. The CO evolution of these models is qualitatively similar 