Kuroba Kaito= the surname means "black feather" and apparently it's meant to refer partly to how the police see him, as some kind of evil character who flies, but also to be a contrast to his true nature.
Hakuba Saguru= surname meaning "white horse", making him out as a knight, and the colour is the antithesis to Kaito.
Koizumi Akako= red because she's fiery, passionate, alluring, but also because of the Lucifer connection, red being the popular colour of Satan.
Nakamori Aoko= blue partly being an opposite colour to Akako, and also as sort of a joke apparently, blue being a colour that is normally associated with calm and Aoko being... not.
There's more, I just can't be bothered to think of them all right now








