We address the metal–organic frameworks UiO-6x (x = 6, 7, 8), their band gaps, and the changes in the band gaps upon perturbations in the metal–organic framework structures. Computational studies were performed with complementary experimental band gap measurements. Band gap modulations upon hydrogen substitutions by NH2 and NO2 on the organic linker, hydroxylation and dehydroxylation of the metal center, different linker lengths (x = 6, 7, 8), and Ti and Hf substitutions for Zr were analyzed in detail. The origin of the band gap changes was thoroughly investigated, and this work confirmed a reduction in the band gap upon NH2 and NO2 substitutions. Furthermore, this work explicitly illustrated that changes in the band gap were also observed by changing the coordination around the Zr atom, whereas isovalent substitutions on the metal center did not yield significant perturbations of the band gap.