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We present new measurements of the UV spectral slope $\beta$ for galaxies at$z=6-9$ in the Frontier Fields cluster MACSJ0416.1-2403 and its parallel field,to an unprecedented level of low stellar mass. We fit synthetic stellarpopulation models to the observed spectral energy distribution and calculate$\beta$ by fitting a power law to the best-fit spectrum. With this method, wereport the derivation of rest-frame UV colours of galaxies for the FrontierFields program extending out to $z=9$, probing magnitudes as faint as$M\mathrm{_{UV}=-13.5}$ at $z=6$. We find no significant correlation between$\beta$ and rest-frame UV magnitude $M_{1500}$ all redshifts, but we do find astrong correlation between $\beta$ and stellar mass with lower mass galaxiesexhibiting bluer UV slopes. At $z=7$ the bluest median value of our sample isredder than previously reported values in the literature, whereas at $z=9$ ourbluest data point has a median value of $\beta=-2.63_{-0.43}^{+0.52}$. Thus, wefind no evidence for extreme stellar populations at $z>6$. We also observe astrong correlation between $\beta$ and SFR, such that galaxies with low SFRsexhibit bluer slopes. Additionally, there exists a star formation main sequenceup to $z = 9$ with SFRs correlating with stellar mass. All of these relationsshow that $\beta$ values correlate with a process that drives both the overallSFR and stellar mass assembly. Furthermore, we observe no trend between $\beta$and specific SFR, suggesting that $\beta$ is getting set by a global processdriven by the scale of the galaxy.

UV Spectral-Slopes at $z=6-9$ in the Hubble Frontier Fields: Lack of Evidence for Unusual or Pop III Stellar Populations