High brightness electron sources for next-generation FELs and ultrafast electron-beam applications will ultimately be limited by the photoemission properties of the cathodes operated in modern RF photoinjectors. Alkali antimonide photocathodes such as Cs${}_3$Sb offer high quantum efficiency at visible wavelengths and low intrinsic emittance when characterized under low fields and ideal vacuum conditions, yet their performance under the harsher environment of high-gradient RF guns remains poorly understood. Here we report a comprehensive wavelength-dependent characterization of two Cs${}_3$Sb photocathodes operated in the UCLA Pegasus 1.6-cell S-band gun with a peak field of 80 MV/m. Using a tunable 260–700~nm drive laser, we measure quantum efficiency and mean transverse energy via charge- and solenoid-scan techniques across a broad range of excess photon energies. RF phase scans are further used to infer the Schottky-modified emission barrier and to demonstrate a proof-of-principle reconstruction of an effective initial longitudinal-energy distribution. Although the absolute quantities reported here depend on the surface history which included re-cesiation and transport-induced degradation, their relative spectral trends provide internally consistent information on photoemission from alkali-antimonide cathodes in an high gradient RF-gun, establishing an experimental framework for benchmarking photoemission models under realistic photoinjector operating conditions.