Colorimetric Behaviour of Ceramic Zirconia Restorations Cemented on Darkened Substrates—In Vitro Study

The colour matching of ceramic restorations is sensitive to ceramic thickness, ceramic optical properties, the tooth region, the tooth/substrate basis colour, and the shade of the bonding agent. This in vitro study evaluates the influence of substrate darkening, resin cement shade and zirconia thickness on the final colour of monolithic Prettau®2 zirconia restorations. An in vitro factorial design was used combining four resin substrates simulating increasing darkening (ND6–ND9), three shades of dual-cure resin cement (universal, transparent, white opaque) and three zirconia thicknesses (0.5, 1.0, 1.5 mm) of Prettau®2 zirconia. Standardized photographs were taken under controlled conditions, and CIELAB coordinates (L*, a*, b*) were obtained in Adobe Photoshop. Colour differences relative to the Prettau®2 A1 shade tab were calculated as ΔL*, Δa*, Δb* and ΔE*. An additive linear model on ΔE* and a main-effect MANOVA on ΔL*, Δa* and Δb* were fitted to assess the impact of each factor. The mean ΔE* was 6.67 ± 2.66, and all but two specimens showed a clinically perceptible colour difference (ΔE* > 2.7) from the A1 shade tab. Substrate shade accounted for 38.4% of the explained variance in ΔE*, cement for 27.6% and zirconia thickness for 6.7%. MANOVA confirmed significant multivariate effects of substrate and cement, but not of zirconia thickness. Translucent monolithic zirconia showed limited ability to reproduce the A1 reference shade over darkened substrates. Substrate shade was the main determinant of colour mismatch, followed by resin cement, whereas zirconia thickness within 0.5–1.5 mm played a minor role. White opaque cement reduced ΔE* and brought the final shade closer to A1, but residual mismatches often remained clinically relevant. These findings highlight the need to control and, when possible, modify the underlying substrate and to select high-opacity cements when shade matching is critical.