Why are onion skin cells colorless?

In fact, the cells themselves are colorless. Various colors (eg red, green) are given to them by pigments. They are found in plastids (for example, chlorophyll, which gives green color, is found in chloroplasts). In turn, plastids may not contain pigments (for example, leukoplasts).
The bulbs are underground, where sunlight cannot penetrate. The cells of their epidermis do not contain pigments, because chlorophyll is necessary for photosynthesis, and it needs light, the main part of the cell is occupied by a vacuole.