## 2 Materials and Methods  
A total of six different RLAs were investigated: the light-cure Nexus Third Generation (NX3L) (Kerr, Orange, CA, USA), dual-cure Nexus Third Generation (NX3D) (Kerr, Orange, CA, USA), light-cure RelyX Veneer (RXL) (3M ESPE, St. Paul, MN, USA), dual-cure RelyX Ultimate (RXD) (3M ESPE, St. Paul, MN, USA), light-cure Enamel Plus HRi Flow (MF) (Micerium, Genova, Italy) and preheated light-cure Enamel Plus HRi (MH) (Micerium, Genova, Italy). The preheated composite MH was obtained by heating a compile of high-viscosity resin composite for 10 min at 55 \({}^{\circ}\)C in an Ena Heat composite heater (Micerium, Genova, Italy) by following the manufacturer's instructions. The compositions of all tested materials are described in Table 1[17]. For each tested RLA, 10 aliquots of unpolymerised material were withdrawn, placed inside a Teflon ring (0.2 mm in height and a 15.0-mm internal diameter) on a Kaltek glass (1.1-mm thick) and covered by another thin glass (0.2-mm thick), thus obtaining a disk-shaped sample of the material with a diameter of 15.0 mm and a thickness of 0.2 mm. A polymerised composite disk made by Filtek Supreme A3B Plus (3M ESPE, St.Paul, MN)that was 2.0-mm thickn and 25.0 mm in diameter was interposed between the tip of the lamp and the resin cement sample during the curing phases [17]. Then, the samples were randomly divided into two groups (n = 5) according to the curing protocols:  
1. Traditional curing protocol (P1), where the samples were cured for 40 s;
2. Step luting protocol (P2), where the samples were cured for 5 s and then, after 20 s, cured again for an additional 40 s [17]. In both curing protocols, an Elipar DeepCure S light (3M ESPE, Seefeld, Germany) was adopted with an irradiance of 1470 mW/cm\({}^{2}\)\(\pm\) 20% and a spectrum range between 430 and 480 nm. The colour parameters were recorded by SpectroShade-Micro (MHT S.p.a., Verona, Italy) using the CIELab and CIELCh systems. CIELab consists of the parameters of the lightness (L*), red-green chromatic coordinate (a*) and blue-yellow chromatic coordinate (b*), while CIELCh consists of the parameters of the lightness (L*), chrome (C*) and hue (h*). The colour evaluations were performed 2 weeks after the curing procedures to have the maximum degree of conversion for each RLA [17]. The colour parameters were measured before (t0) and 1 day (t1) and 7 days (t7) after immersion of the samples in distilled water (Figure 1). The colour of each sample was recorded against white (L* = 95.00, a* = -0.07, b* = -4.20) and black (L* = 9.20, a* = 1.47, b* = -3.83) backgrounds. Moreover, each measurement was repeated three times in the centre of each sample without replacement to increase the accuracy. During the experiment, the samples were kept at room temperature, in constant humidity and in the absence of light. The colour variation (\(\Delta\)E) was calculated using CIELab values on a white background:  
\[\Delta\mathrm{E(t0/t1)^{*}} =[(\mathrm{L_{t0}}^{*}-\mathrm{L_{t1}}^{*})^{2}+(\mathrm{a_{t0}}^ {*}-\mathrm{a_{t1}}^{*})^{2}+(\mathrm{b_{t0}}^{*}-\mathrm{b_{t1}}^{*})^{2}]^{ 1/2} \tag{1}\] \[\Delta\mathrm{E(t0/t7)^{*}} =[(\mathrm{L_{t0}}^{*}-\mathrm{L_{t7}}^{*})^{2}+(\mathrm{a_{t0}}^ {*}-\mathrm{a_{t7}}^{*})^{2}+(\mathrm{b_{t0}}^{*}-\mathrm{b_{t7}}^{*})^{2}]^{ 1/2} \tag{2}\]