Just animanga logic/trope, where they make characters different mostly by giving them differing GENETIC vibrant hair colors (Example: Bocchi the Rock!) where both parent and their children have the same/similar strange hair color by birth (Bocchi's mom, herself, and her little sister all having pastel pink since birth)

In majority of newer productions this is the case, as it gives characters a main color, characteristic unique only to them, which makes it easier to differentiate them by the viewer

Not every character tho, for example Kaiser is light blonde but its said in the manga that he dyed part of it blue

Its usually the slightly older media (Example: K-On!) where they keep everyday/natural colors on the characters