Both PHYs, also known as phytonutrients, and nutraceuticals are bioactive compounds that can be found in edible products possessing beneficial properties capable of enhancing human health and are commonly ingested with the diet. Anyway, while nutraceuticals are essential nutrients for human life and are necessarily present in edible products derived from animals, plants, or fungi, PHYs are non-nutrients and exclusively originate from plants [17]. Although a lower intake of PHYs does not cause defects, these compounds can be invaluable to human health, and a diet rich in PHYs is strongly connected to better health [18]. In vitro studies have demonstrated that PHYs are multifunctional compounds with health-promoting properties like those of conventional drugs and can be considered “pharmaceutical-grade compounds”. They can improve the ability to detox, boost the immune system, and help to protect against age-related diseases, such as diabetes, heart disease, and osteoporosis [18].

It has been reported that scientists have already identified over 5000 different classes of PHYs. Anyway, many more remain undiscovered, and much more has to be learned about their potential benefits [19]. Although PHYs can derive from both edible and non-edible plants [20], all plant-based foods, including fruits, vegetables, nuts, and herbs, contain them. Generally, PHYs confer to the plant-derived food containing them a particular bright color, but foods without these characteristics can contain these healthy promoters, as well (Table 1). Green, purple, red, blue, or yellow vegetables and fruits contain colored PHYs [19,21,22], but non-brightly colored potatoes, cauliflower, nuts such as almonds, cashews, and hazelnuts, tea, and dark chocolate also contain several PHYs (Table 1) [19,23,24,25]. Moreover, essential oils (EOs), including those from pine needles, cedar, and lavender, are used as health promoters due to their PHY contents (Table 1) [26,27,28].

The most common PHYs are polyphenols, carotenoids, coumarins, indoles, organosulfur compounds, isothiocyanates, saponins, tannins, phenylpropanoids, anthraquinones, ginsenosides, terpenoids, etc. [39]. In Table 2 below, the most relevant PHYs, their sources, and the associated health-promoting effects were reported [3,40].

As mentioned above, the known PHYs are in the thousands. In Figure 3 and Figure 4, the chemical structures of the most common polyphenols (Figure 3) and of other relevant PHYs (Figure 4) with their properties [1,3,10,11,17,18,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65] were reported.

Since PHYs confer particular food colors, colored foods surely contain PHYs and possess the same benefits as PHYs having the same color. So, by eating foods from all of the different color groups, a wide range of different PHYs can be assumed, with a consequent broad spectrum of benefits [66]. Table 3 below reports the general subdivision of plant-related foods into the different color groups.