An effective method to produce well-defined polymer brushes with high spatial, temporal, and sequence control is to employ a photoredox catalyst in photo-mediated polymerization. Generally, the excitation wavelength is chosen as the absorption maximum of the photocatalyst, however, it is not clear if that corresponds to the best photochemical activity for producing polymer brushes. Herein, we systematically examine wavelength-by-wavelength resolved polymer brush growth using surface-initiated photoinduced electron transfer-reversible addition-fragmentation chain transfer polymerization (SI-PET-RAFT), of four monomer types. The absorption spectra of the water-soluble photocatalyst (ZnTPPS₄⁴⁻) and the brush growth at each irradiation wavelength were examined as photochemical activity plots. Our results show a striking disparity between the absorbance of the photoredox catalyst and the maximum brush height. Moreover, the photochemical activity with wavelength was highly dependent on the nature of the monomer used. In addition to displaying a strong wavelength selectivity, a characteristic red-shift in the brush height relative to the lowest possible energy transition of the photocatalyst's absorbance spectrum was observed. We anticipate this work will better inform on wavelength choice for SI-PET-RAFT polymerization of polymer brushes.