The present study was designed to investigate the influence of different gingival colors, restoration thicknesses, and substrate colors on the masking ability of GCRBCs. The null hypothesis was rejected based on the results revealing significant interactions of gingival color, restoration thickness, and substrate color on both ∆E*ab and ΔE00 values of GCRBCs.

Color and appearance have been shown to be the key factors in determining the success of restorations in esthetic dentistry [8, 16, 17]. However, research related to color has been focused on teeth and related restorative materials leaving the importance of gingival color non-emphasized [8, 16, 17]. Several studies have been conducted to find out the complex balance of factors effective on the final color of restorations [1, 7, 18]. Clinically relevant testing of color perception can help to better understand the complex nature of color and factors effective on the final color of GCRBCs [7, 8]. In the present study, five different gingival colors with 2 different thicknesses were placed over 3 different tooth-colored substrates. For this purpose, tooth-colored composite material designed for simulating different tooth shades was used. The use of tooth-colored substrates as backgrounds could provide a clinically reliable approach for evaluating color differences rather than black and white backgrounds as suggested by the International Organization for Standardization, ISO/TR 28642–2016 [7, 13].

The color of GCRBCs could be visually or instrumentally assessed using spectrophotometers or spectroradiometers [1, 2, 6, 7]. In the present study, a spectroradiometer was used for color measurements. Spectroradiometers provide noncontact color measurements and unlike spectrophotometers do not contain a stable light source [2]. In addition, color measurements with spectroradiometers can be achieved under the same viewing conditions as human observers [17, 19]. Different color systems and formulas are used to assess the color of restorations recognized by the human eye [10,11,12]. Traditionally, the CIELab color difference formula is used for the assessment of perceptibility and acceptability of color differences [17]. However, CIELab color space is reported to be non-uniform and CIEDE2000 color difference formula is recommended to achieve a better correlation with visual perception [10,11,12]. In the present study, the results of color differences were presented using both CIELab and CIEDE2000 color difference formulas. For the interpretation of color differences, PT and AT visual thresholds were used to correlate the numerical data with what is perceived and visualized clinically [2].

In the present study, ∆E*ab and ΔE00 values of GCRBCs were significantly affected by different gingival colors, thicknesses, and substrate colors. Significantly higher ∆E*ab and ΔE00 values were found in the LP-1.0 mm and Or-1.0 mm groups layered over ND9 substrates that presented values considered as moderately unacceptable. GCRBC thickness influenced ∆E00 values in the gingival color groups of LP, DP, Or, and Br when layered over ND9 substrates. Higher color differences were found in the GCRBC groups (LP, DP, Or, and Br) with a thickness of 1.0 mm when compared to the thickness of 2.0 mm. A recent study by Pérez et al. compared the influence of different background colors and thicknesses on the appearance of gingiva-colored composite resins. In accordance with the present study, they reported significantly higher color differences for the restorations with a thickness of 1.0 mm [8]. In the present study, color measurements obtained over ND9 substrates showed higher ∆E*ab and ΔE00 values than the measurements obtained over ND1 and ND5 substrates for the gingival color groups of LP, DP, Or, and Br. More specifically, changing the gingival color from a lighter pink to a darker purple color resulted in decreased ∆E*ab and ΔE00 values. Furthermore, thickness and substrate color did not significantly influence the color differences in the gingival color groups of P. Recently, Gouveia et al. investigated the influence of different background colors and thicknesses on the final color of gingiva-colored composites. In line with the present results, the authors concluded that background color and gingiva shade influenced the final color of gingiva-colored composites [9]. In addition to the studies on GCRBCs, several studies evaluating visual thresholds of tooth-colored restorative materials have been conducted [7,8,9]. The research on restorative materials has shown the effect of several factors on color perception, such as environmental background, illumination source, substrate color, restoration thickness, cement shade and measurement technique [7,8,9, 18, 20]. It has been reported that the color of background and restoration thickness influence the color differences of restorative materials [7, 9].

As a limitation of the present study, only one brand of GCRBC material was used. Additionally, color measurements were performed using only instrumental color measurement device. Further research of masking ability is needed, accompanying instrumental color measurements with human visual assessments, which could help to confront the visual thresholds and to have a better perceptive information about the clinical acceptability of GCRBCs. In vivo studies assessing the effect of different material brands, gingiva, and substrate colors would also be useful to verify the blending effect of several parameters on the color perception of GCRBCs.