Enhanced coupling to slow photon modes at the band-edge of graded three-dimensional, thin-film polymer colloidal photonic crystals (see Figure) is demonstrated. For the gradation, polystyrene latex is etched by exposure to air plasma, yielding a top-to-bottom filling-fraction gradient. By monitoring transmission and corresponding phase spectra upon etching, spectral blue-shifts and enhanced mode coupling are observed. Also, surface roughness is reduced, resulting in higher structural and optical quality colloidal photonic crystals.