## 1 Introduction  
Modern restorative dentistry is based on the capability of resin-based materials (RBMs) to bond dental tissues. RBMs allow reconstructing dental tissues lost due to trauma, erosion or dental caries by a simple adhesive and minimally invasive procedures. In particular, in the case of endodontically treated [1, 2] and structurally compromised teeth, an indirect restoration is required to re-establish their functionality and aesthetics [3]. Indirect restorations can be made of different materials and thicknesses, and they are bonded to dental substrates using RBMs, or the so-called resin luting agent (RLA) [4, 5]. RLAs can be divided into two categories: traditional flowable resin-based composites and resin cements [6]. The latter can be divided into light-, self- or dual-cure according to the curing procedure, their monomers and formulation. Following the concept of minimally invasive dentistry, tooth preparation for an indirect restoration aims to save as much dental tissue as possible. This type of preparation often requires a supragingiyal margin, and consequently, it may affect the aesthetic result of the indirect restoration, mainly on the buccal and lingual sides. Therefore, in the area of the preparation margin, which is often visible, three different substrates can be identified, each with its own colour properties: the dental tissue, the material that constitutes the indirect restoration and the RLA. Clinically, the colour of the dental tissue and the material of the indirect restoration are often checked and decided following a colour shade guide or a dental photograph, while the colour of the RLA is rarely checked or evaluated previously. Different try-in gels were proposed from the manufacturer to check the colour of the RLA, but they might not match their final colour after curing [7]. Although the thickness of the cement layer is around 40 \(\upmu\)m [8], the choice of the exact colour of the RLA can be fundamental to hiding the preparation margin and \"linking\" the colour of the indirect restoration to the surrounding dental tissues. The colour of an object can be recorded by a spectrophotometer and described using the parameters expressed by the Commission Internationale d'Eclairage (CIE), namely with the CIELab and CIELCh systems [9; 10]. These systems allow one to calculate both \(\Delta\)E, which quantifies the difference between two colours, and the translucency, which describes the behaviour of an object for transmitting or diffusing light. Translucency has been emphasised as one of the most important properties of RBMs, since it favours the mimesis of the material with the surrounding dental tissues [11; 12]. Translucency can be calculated using the translucency parameter (TP) and contrast ratio. These values are highly correlated and can be used interchangeably [13; 14]. The colour and translucency of all RBMs can change over time due to several factors, including storage in water, chemical initiators (such as tertiary amine, benzoyl peroxide or camphorquinone) and the curing protocol [13; 15]. In clinical practice, RLAs undergo the curing process, which can influence the chemical and physical properties, including colour and translucency [16]. Both light and dual-cure RLAs can be cured with different protocols, such as the traditional or step luting protocols [17]. The traditional curing protocol involves the polymerisation of RLAs in a single step. Instead, the step luting protocol consists of curing the RLA for 5 s, waiting around 20 s to remove excesses due to the indirect restoration placement, and curing again. The 5-s step allows the easy removal of excesses thanks to the gel phase of RLAs, in which the materials are still soft [17]. Some studies evaluated the colour and translucency of RLAs along with indirect restorations [4; 18], but there is no research on the evaluation of RLAs alone. In addition, although the effect of the step luting protocol on the degree of conversion was estimated, no study described its effect on the colour properties.