Comparative Analysis of 24-Hour Post-Polymerisation Colour Change in Experimental Rice Husk Silica-Incorporated Flowable Composites with Different Photoinitiator Systems Across Three Colour-Assessment Devices

Background/Objectives: Rice husk silica (RHS) is a biosustainable filler with potential application in dental flowable composites (FCs). However, early post-polymerisation colour change in RHS-incorporated FCs may vary with material formulation, particularly resin composition and photoinitiator system, and may also be affected by the colour-assessment device used. This study compared the 24 h post-polymerisation colour change of 10 experimental RHS-incorporated FC groups formulated with five photoinitiator systems and two commercial FCs used as control, using three colour-assessment devices. Methods: Two UDMA/TEGDMA formulations (U50 and U40), each containing 50 wt% nanohybrid RHS, were prepared with different photoinitiator systems (CQ, PPD, CQ/PPD, BAPO, or TPO). CIELAB coordinates were recorded immediately after curing and after 24 h of dry, dark storage at 37 °C using D1 (VITA Easyshade), D2 (Ds-700d Portable Spectrophotometer), and D3 (ColorMeter Pro). The same specimens were measured with all three devices, with five matched specimens from each of the 12 composite groups (N = 60). A mixed repeated-measures ANOVA was performed with device as the within-subject factor and composite as the between-subject factor; agreement was additionally assessed using Bland–Altman analysis and an absolute-agreement intraclass correlation coefficient (ICC). Results: Device, composite, and the device × composite interaction were significant (all p < 0.001). Estimated mean ΔE*ab was highest for D1 (5.019), followed by D3 (2.368) and D2 (1.777), and all device pairs differed significantly after Bonferroni adjustment. Bland–Altman analysis showed systematic positive bias for D1 relative to D2 and D3. The absolute-agreement ICC was 0.130 (95% CI 0.078–0.177), indicating poor agreement. Conclusions: The magnitude of early 24 h colour change depended on both the composite material and the measurement device. The three devices should not be assumed to be directly interchangeable for ΔE*ab assessment; the same calibrated device should be used throughout a longitudinal comparison.