Abstract We present new measurements of the UV spectral slope β for galaxies at z = 6−9 in the Frontier Field cluster MACS J0416.1–2403 and its parallel field to an unprecedented level of low stellar mass. We fit synthetic stellar population models to the observed spectral energy distribution and calculate β by fitting a power law to the best-fit spectrum. With this method, we report the derivation of rest-frame UV colors of galaxies for the Frontier Fields program extending out to z = 9, probing magnitudes as faint as M UV = −13.5 at z = 6. We find no significant correlation between β and rest-frame UV magnitude M 1500 all redshifts, but we do find a strong correlation between β and stellar mass, with lower-mass galaxies exhibiting bluer UV slopes. At z = 7, the bluest median value of our sample is redder than the previously reported values in the literature, whereas at z = 9, our bluest data point has a median value of β = − 2.63 − 0.43 + 0.52 . Thus, we find no evidence for extreme stellar populations at z > 6. We also observe a strong correlation between β and star formation rate (SFR), such that galaxies with low SFRs exhibit bluer slopes. Additionally, there exists a star formation main sequence up to z = 9 with SFRs correlating with stellar mass. All of these relations show that β values correlate with a process that drives both the overall SFR and stellar mass assembly. Furthermore, we observe no trend between β and specific SFR, suggesting that β is getting set by a global process driven by the scale of the galaxy.