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We present measurements of the rest-frame UV spectral slope, $\beta$, for asample of 36 faint star-forming galaxies at z ~ 9-16 discovered in one of thedeepest JWST NIRCam surveys to date, the Next Generation Deep ExtragalacticExploratory Public (NGDEEP) Survey. We use robust photometric measurements forUV-faint galaxies (down to $M_{UV}$ ~ -16), originally published in Leung+23,and measure values of the UV spectral slope via photometric power-law fittingto both the observed photometry and to stellar population models obtainedthrough spectral energy distribution (SED) fitting with Bagpipes. We obtain amedian and 68% confidence interval for $\beta$ from photometric power-lawfitting of $\beta_{PL} = -2.7^{+0.5}_{-0.5}$ and from SED-fitting, $\beta_{SED}= -2.3^{+0.2}_{-0.1}$ for the full sample. We show that when only 2-3photometric detections are available, SED-fitting has a lower scatter andreduced biases than photometric power-law fitting. We quantify this bias andfind that after correction, the median $\beta_{SED,corr} = -2.5^{+0.2}_{-0.2}$.We measure physical properties for our galaxies with Bagpipes and find that ourfaint ($M_{UV} = -18.1^{+0.7}_{-0.9}$) sample is low mass(${log}[M_{\ast}/M_\odot] = 7.7^{+0.5}_{-0.5}$), fairly dust-poor ($A_{v} =0.1^{+0.2}_{-0.1}$ mag), and modestly young (${log[age]} = 7.8^{+0.2}_{-0.8}$yr) with a median star formation rate of $\mathrm{log(SFR)} =-0.3^{+0.4}_{-0.4} M_\odot{/yr}$. We find no strong evidence for ultra-blue UVspectral slopes ($\beta$ ~ -3) within our sample, as would be expected forexotically metal-poor ($Z/Z_{\odot}$ < 10$^{-3}$) stellar populations with veryhigh LyC escape fractions. Our observations are consistent with modelpredictions that galaxies of these stellar masses at z~9-16 should have onlymodestly low metallicities ($Z/Z_{\odot}$ ~ 0.1--0.2).

Rest-Frame UV Colors for Faint Galaxies at $z \sim 9-16$ with the \textit{JWST} NGDEEP Survey