axel

harpuia

his primary colour is green. green is often associated with peace, nature, new beginnings, life, gentleness, renewal, and growth. this suits his character extensively; he is one of the biggest examples of growth in the series, by going from being intensely loyal to copy x to rebelling for proper justice. he even showed mercy to zero in the beginning of z2 instead of taking the opportunity to quickly and easily kill him, setting down the basis for his more peaceful ideals. not to mention his first name being sage, a shade of green.

fefnir

his primary colour is red. red is often associated with harshness, danger, passion, intensity, and courage. fefnir has repeatedly displayed a passion for fighting (zero specifically). he's not afraid to do the hard work, even if he doesn't want to (such as dropping the bomb at the resistance base). it shows he has strong courage and harshness.

levithan

her primary colour is blue. blue is associated with wisdom, sincerity, imagination, loyalty, enthusiasm, and sadness. much like the others, she is extremely loyal to copy x. she's also very sincere, with her blatant honestly with her desire to kill zero and how fighting him feels better than fighting others (specifically omega). i find her personality slightly contrasts her colour themes, but they fit with her preference to be aquatic.

phantom

his primary colour is black. this is often associated with death, power, restraint, grief, and elegance. phantom was very extreme when it came to his goals, with him even being the first to die for the sake of neo arcadia. he was merciless and powerful and fought zero with the quickest approach of the four. he initially held proper restraint as well, taking precaution before fighting zero, unlike fefnir and levithan.

overall

they all wear white under their main armour, the colour often associated with purity. it's a reminder that they were created for the aid of settling humans initially, despite where things evolved.