## 4 Conclusions  
Based on our experience and after doing a wide literature review, we propound a general protocol to optimize a simple natural dye production method, without complicating the chemical procedures, with the aim of enhancing the dye potential to whatever subsequent use. In particular, as a possible application, NDSSCs performance is considered. In conclusion, the protocol consists of these main and minimum advisable steps to obtain and test natural pigments: (i) Dye solutions preparation and characterization by UV-Vis spectrometry: Select natural raw vegetable pigment, preferably to be obtained locally and easily, and select the simplest and best extraction solution and method. (ii) Exam dye solutions behavior i.e., how they adsorb onto semiconductor film, how stable they are, or what is the influence of different parameters on extraction and adsorption. And finally (iii) NDSSC tests: As an example of application of the previous exhaustive study, we have designed a simple and economical cell fabrication method and measured cell performance in environmental conditions varying the dye and with all the other parameters fixed. The novelty of this approach is that it gives a previous deeper knowledge of the dye, crucial to the final NDSSC performance. We have successfully employed this protocol with betalain pigments (a series of four bougainvillea dyes and three beetroot dyes) and anthocyanins (three eggplant dyes). The previous UV-Vis spectra analysis gives useful information about:  
* The best extraction solution and method: for betalains, Solution 1 (acetone/water, moderately acid), for anthocyanins, Solutions 2 and 3 (very acidic solutions). * The relative concentration of different pigment molecules for the different extracts. * Some candidate vegetable material can be a priori selected or ruled out by inspection of the extract spectra. Besides, comparison with the corresponding spectra of dry raw material performed with a spectroradiometer, gives reliable information about the dye molecules which can be potentially extracted. We show the example of bougainvillea-colored bracts spectra. * The dye stability or ageing: Anthocyanins are more stable and betalain molecules degrade, but conservation of the dry precursor (collected bracts) for at least 6 months is possible. * How the dye adsorbs onto photoelectrode film, and the effect of mixing dye extracts. In this paper, we focus on mixing dyes from the betalain family to complement results in the bibliography. For the selected cell fabrication method, NDSSC tests show that for anthocyanins the best performances were obtained with Solutions 2 and 3, but with very low values compared to literature. In this work, betalains give higher efficiencies than anthocyanins. For betalains, the best extract is obtained with Solution 1, as initially found from UV-Vis spectrometry analysis. Within the betalain family, for bougainvillea dyes, similar performances were obtained for the different varieties, and for beetroots the best result reached up to 0.47% cell efficiency, which is among the highest values in literature. Conceptualization, M.J.G.-S.; Methodology, M.J.G.-S.; Investigation, M.J.G.-S.; Resources, M.J.A. ; Writing--Original Draft Preparation, M.J.G.-S.; Writing--Review & Editing, M.J.G.-S. and M.J.A. ; Visualization, M.J.A. ; Supervision, M.J.G.-S. and M.J.A. ; Project Administration, M.J.A. ; Funding Acquisition, M.J.A. Both authors have read and approved the manuscript. The research work leading to this article received ERDF funds from the Spanish government within the framework of the SOLTERMIN project (Ref. ENE2017-83973-R) of the Ministerio de Economia, Industria y Competitividad (Spanish Ministry of Economy, Industry and Competitiveness). The authors sincerely thank [PERSON] and [PERSON] from CIESOL (Solar Energy Research Center, UAL-PSA, Almeria, Spain); and [PERSON] from Plataforma Solar de Almeria (PSA-CIEMAT, Almeria, Spain); [PERSON] and [PERSON] are acknowledged for the technical support. \"The authors declare no conflict of interest.\".