**1. Introduction**

The latest surveys on the current energy use scenario in Europe reported that buildings account for almost 40% of energy consumptions and 36% of carbon dioxide emissions (CO2 emissions) [1]. The European Union (EU) has adopted a comprehensive regulatory framework to meet the commitments stated in the Paris Agreement [2] and to facilitate the transition from fossil fuels to cleaner energy production. The Energy Performance of Buildings Directive (EPBD) and the European Directive on Energy E fficiency (EED) provide the roadmap for the transformation of the existing building stock into nearly zero-energy buildings (nZEB) [3,4] through the definition of specific measures to improve the energy e fficiency of buildings, reduce the energy use and enhance the decarbonisation in the construction sector. Furthermore, the recast version of the Renewable Energy Directive [5] set the target for renewable energy source (RES) penetration in the European energy mix to 32% by 2030. Energy flexibility in buildings is, moreover, gaining momentum through the introduction of self-consumers and collective self-consumers concepts in regulations which, on the one hand, empower users to be prosumers instead of merely consumers and, on the other hand, enable buildings to manage their energy demand and production, by storing, consuming or selling electricity according to their need [5,6]. In this context, building integrated photovoltaics (BIPV) technologies can support the transition towards a low-carbon energy system, promoting on-site energy production and enhancing self-consumption, if integrated into the overall building/district energy system and coupled with electric or thermal storages [7]. BIPV constitute indeed a solution to incorporate RES in the built environment by integrating solar photovoltaics (PV) technologies in the building envelope. More precisely, BIPV systems have a dual purpose: they serve as building envelope and as power generation systems at the same time, harvesting solar energy for on-site energy production [8]. This is a relevant characteristic, especially for the decarbonisation of energy systems in densely built environments, where the on-site energy production is difficult to exploit due to urban constraints, which hinder traditional ground-mounted PV installation. In addition, the solar potential for roof-mounted PV is low if compared to multi-property and multi-story building energy demand [9]. Despite the high potential for BIPV applications, there is the need to overcome technical, social and economic barriers to reach a larger scale of BIPV applications, improving their economic profitability [10–12]. Besides the cost, which has consistently decreased recently, the main limit to the spread of BIPV has so far been their aesthetic, since they are often considered anaesthetic by users and architects [13]: "[ ... ] *When we hear about photovoltaics, however, the image that is invoked in our mind is a blue or black element that usually seems to "overload" the aesthetic image of a building* [14]. Research institutes, universities and industries are working together to design and produce a novel generation of BIPV solutions which will transform the visual appearance of standard PV modules into a more "architecturally pleasing" one [15] to be integrated into sensitive environments. Crystalline silicon modules, thin films, coloured solar cells, homogenised black appearance and integration of high-resolution images are just a few examples of the new possibilities offered by the PV market. However, the turning point in the acceptance of BIPV applications has been the development of hidden coloured BIPV modules. This module typology (which includes several different technologies, as discussed in Section 4) can hide the PV cells behind coloured patterns which hinder the perception of the original material of the cells. In this way, the modules appear very similar to standard construction materials [9]. As a result, a wide range of colours is currently available on the market, and this wide selection enables PV to be integrated also in traditional roofs, façades, and shading systems (Section 4). Nonetheless, the production of these modules is predominantly customisable for a specific installation. Modules customisation ensure a variety of new aesthetic and technical possibilities that facilitate the use of BIPV technologies also in densely built environments. This solution appears appropriate also for architecturally sensitive areas (i.e., historical centres, vernacular and historic buildings, natural and cultural landscapes), thanks to the aesthetical and technological advances related to low-rate reflection, mimetic appearance, compact shape and geometric flexibility [16–19]. Thus, these aesthetic improvements unlock the solar potential of a large set of vertical and horizontal envelope surfaces currently not exploited, leading the building stock to energy flexibility and self-sufficiency [20,21]. Conversely, the extreme requirements of customisation results in a fragmented market scenario and a high variety of colours, shapes, sizes, finishing, mechanical robustness and electricity generation efficiencies. Therefore, there is an urgen<sup>t</sup> need to better frame the hidden coloured BIPV technologies available on the market, while building a common dialectic within the stakeholders along the entire value chain, to boost the BIPV market penetration. Section 2 indicates the aims and methodology pursued in this paper and outlines its structure.
