Guren's fav colour is bluish-grey (if I'm not wrong), which undoubtedly is mahiru's hair colour, but I read it somewhere that it also relates to shinya. Also many say that shinya's hair is silver so I wanted to know is shinya's hair colour silver(grey) or white.

(Both ways I will be disappointed)

i honestly don't think it refers to either of them. his answer does say "i don't have a particular reason for it", doesn't it?

but well, while mahiru's hair is described as ashen (so pretty much grey), most of the time it's painted in a sort of purplish or magenta colour - i don't think it's supposed to be blue.

and shinya, the weird shade of dirty blonde they used in the anime aside, is supposed to have "striking snow white" hair. so like, the whitest of white, probably. i don't think hair can be 'silver' at all, but i've also seen it used to describe his a lot. nevertheless, any sort of colour on there is just going to be caused by lighting and environment. the only thing about him that's really blue is his eyes. 🧿👄🧿

... and perhaps his face when seishirō is having a bad day.

but well, if it is supposed to be a nod to someone, then it's probably mahiru. iirc his "what are you interested in in the opposite sex" answer literally just said "mahiru hiiragi" as well.