Water is green.
Fire is blue.
Earth is red.
Why?
this comes from the religion of the world and how it's made. this world of magic has it's influance based in religion and the gods actions despite it going away from it. it was actually explained why at the start of s2 but it's just as good an explenation as others. TLDR colors are associated with their respective gods their domains and the countries foundational myths.
Flutrane the godess of water is associated with spring and the coming of the change (like the changing of the seasons) it's colors are then associated with the things incorporated into that season for spring that's the melting of the snow and the start of new GREENery in the form of plants.
this goes on for all the primary gods. and thus their associated colors.
Leidenschaft-> Fire->summer (strongest god to drive away winter)-> clear blue skys often seen in Middle European lands (the geographical setting of Ascendance of a bookworm even if the world is magical)
Schutzaria -> wind -> Fall -> highly associated with the winds and harvest as the main product of the country is wheat the color follows suit -> yellow for the harvested wheat
Winter is best described with the 2 gods that are both associated with it and their differing personalities
Ewigeliebe -> life -> winter -> color white is associated with snow and a reflection of his possessiveness for his wife which he covered in ice(a real yandere here)
Geduldh ->earth -> winter -> color red is associated with the Hearth (fireplace) a reprieve from the harshness of winter and accepting to all who come near it (reflective of her accepting personality of all ppl)
lastly there are 2 more who don't have a name for all ppl and aren't associated with seaons.
the god of dakness->associated with the night -> thus his color is Black (also associated with fatherhood and thus his subordinates are those who are associated with fatherly actions)
the godess of light ->associated with the day -> thus her color is the noble Golden color of Gold as the sun. (also associated with motherhood and the preservation of order)