## 5 Conclusions  
The choice of a ceramic system is crucial to the esthetic outcome of restorations and their seamless integration into the oral environment, especially when substrates different from natural tooth color are used.  
Despite the limitations of this in vitro study based on a single tooth experiment, it can be concluded that:  
* The ZZ zirconia crowns exhibited greater color changes than the AG restorations.
* Zirconia crowns placed over metal substrates revealed higher color differences than the reference color.
* In general, zirconia crowns cemented with the neutral cement led to fewer color differences than when the light cement was used.
* Both light and neutral cement influenced the AG and ZZ crowns over the zirconia substrate.
* Both light and neutral cement influenced AG over the natural tooth substrate.
* The neutral cement had the least impact when applied over a metal substrate for the AG crowns or over natural tooth substrates for the ZZ crowns.
* Higher sintering temperatures can enhance translucency but may also increase the restoration's sensitivity to underlying substrates, potentially leading to greater color deviations.  
Potential bias could occur when different crown thicknesses and different tooth preparations are made on different morphological teeth to be restored. To avoid those problems, tooth preparation and crown thicknesses were standardized through the same tooth sample, and equivalent tooth materials from different brands were used. This study should be replicated by varying each of the variables at a different time to confirm the results obtained with an upper first premolar and understand if conclusions can be generalized to other teeth in the mouth.  
Conceptualization, V.F., C.B.N. and J.C.R.; methodology, V.F., C.B.N. and J.C.R.; software, V.A. and J.P.; validation, F.C., C.B.N. and V.A.; formal analysis, C.B.N.; investigation, V.F., C.B.N., J.C.R. and F.C.; resources, C.B.N.; data curation, C.B.N.; writing--original draft preparation, V.F.; writing--review and editing, V.A.; visualization, V.A. and F.C.; supervision, C.B.N. and F.C.; project administration, C.B.N. and F.C.; funding acquisition, C.B.N. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Ethical review and approval were waived for this study due to the use of previously collected human samples that was removed as part of a standard dental care, for reasons unrelated to the study and no additional risk or burden is posed to the individual.  
Informed Consent Statement: Informed consent was obtained from the subject involved in the study.  
The original contributions presented in the study are included in the article; further inquiries can be directed to the corresponding author.  
The authors gratefully acknowledge the support from FCT-Fundacao para a Ciencia e Tecnologia (Portuguese Foundation for Science and Technology) through IDMEC under LAETA Base Funding (DOI: 10.54499/UIDB/50022/2020). In addition, the authors would like to thank the two commercial laboratories that milled the zirconia blocks (AG zirconia block--at the Corus laboratory (Cascais, Portugal), the ZZ zirconia block--at the Hi-Tec laboratory (Oeiras, Portugal)).