## 1 Introduction  
It is logical to consider the 21 st century as the cradle of the digital era. Over the last decade, the proliferation of data display screens has significantly transformed our patterns of work, study, consumption, leisure, and entertainment. The frequency and duration of screen use have escalated considerably over the years. The COVID-19 pandemic has further intensified the global reliance on digital screens across virtually all social sectors. This shift in behaviour and lifestyle habits has led to a substantial increase in daily screen time. Numerous studies have quantified and characterised this rise in screen usage. For instance, [PERSON] et al. [1] found that during the confinement period, 145 adolescents from an Italian region increased their smartphone use by 55%, exceeding four hours daily. In India, [PERSON] et al. [2] observed that the average smartphone usage jumped from 2.4 h pre-pandemic to 6.9 h post-pandemic. Similarly, [PERSON] et al. [3] noted a daily increase of over 120 min in smartphone usage among 22 students in Seville, aged 20 to 36, both before and after the pandemic. On a global scale, [PERSON] et al. [4] highlighted a problematic and exponential increase in mobile device addiction across 24 countries. With the surge in digital screen use, researchers and experts have explored whether prolonged screen time compromises ocular health and vision. [PERSON] et al. [5], in their study of 27,000 individuals under 26, reported that excessive screen use could lead to increased myopia and blurry vision in children. [PERSON] et al. [6] found a direct link between smartphone gaming and dry eye. Numerous other studies have reinforced the idea that excessive screen use can affect binocular vision, increase myopia, cause tear film instability, and induce ocular fatigue, among other issues [7; 8; 9; 10; 11]. Despite the substantial body of research linking screen time to ocular health, there have been no studies directly examining the impact of prolonged screen use on colour vision or the potential increase in acquired colour vision deficiencies. Additionally, the effects of COVID-19 infection on colour vision have not been a primary focus of research. While most studies on ocular manifestations of COVID-19 [12] do not address colour discrimination, [PERSON] et al. [13] noted a potential short-term effect on colour vision, which was transient and reversible. There is a significant gap in the literature concerning the relationship between prolonged screen use and colour vision capacity, as well as the impact of COVID-19 infection on colour vision alterations. Therefore, this study aims to contribute to and expand the knowledge in these under-researched areas. According to [PERSON] and [PERSON] [14], colour vision is the ability to distinguish objects based on the wavelengths they emit or reflect. For colour vision to occur, interactions between several elements are essential: an object, a light source, and an observer (eye and brain) [15; 16]. Understanding the anatomical, physiological, and functional aspects of normal human colour vision is particularly relevant to this study. Normal human colour vision is trichromatic [17], meaning all colours can be created by mixing three primary colours. The photoreceptor cells responsible for trichromatic vision are cones [18]. The human eye has three types of cones: short-wavelength (S-), medium-wavelength (M-), and long-wavelength (L-) sensitive cones [19]. These cones have distinct spectral sensitivities, with peak sensitivities around 419 nm, 531 nm, and 558 nm for S, M, and L cones, respectively [20]. S-cones represent 7-10% of the total, while M and L cones vary widely, with M:L ratios ranging from 1:1 to 1:18 [21; 22]. Each cone contains a type of photopigment that determines its spectral sensitivity. The proportions and types of photopigments are genetically encoded. M and L photopigments are encoded on the X chromosome, while S photopigments are encoded on chromosome 7 [23].