Understanding the nature of light transmission and the photonic band gap in inverse opal photonic crystals is essential for linking their optical characteristics to any application. This is especially important when these structures are examined in liquids or solvents. Knowledge of the true correlation between the nature of the inverse opal (IO) photonic band gap, the structure of the photonic crystals, and the theories that describe their optical spectra is surprisingly limited compared to colloidal opals or more traditional photonic crystal structures. We examined
Ti
O
2
and
Sn
O
2
IOs in a range of common solvents to solve the conflict between Bragg-Snell theory and optical and physical measurements by a comprehensive angle-resolved light transmission study coupled to microscopy examination of the IO structure. Tuning the position of the photonic band gap and index contrast by solvent infiltration of each inverse opal requires a modification to the Bragg-Snell theory and the photonic crystal unit cell definition. We also demonstrate experimentally and theoretically that low fill factors are caused by less dense material infilling all interstitial vacancies in the opal template to form an IO. By also including an optical interference condition for inverse opals with an effective refractive index greater than their substrate, and an alternative internal refraction angle in the substrate, angle-resolved transmission spectra for inverse opals are now consistent with microscopy and refractive index measurements. This work now allows an accurate correlation between the true response of an IO to the index contrast with a solvent, how an IO is infilled, and the directionality and bandwidth of the photonic band gap. As control in functional photonic materials becomes more prevalent outside of optics and photonics, such as biosensing and energy storage, for example, a comprehensive and consistent correlation between photonic crystal structures and their primary optical signatures is a fundamental requirement for application.