Proof that it is not grey/gray:

http://kh2.co.uk/image.php?view=kh/artwork/014.jpg

Notice how here it has a blue hue to it...

http://kh2.co.uk/image.php?view=kh/artwork/023.jpg

See it is silver for it has natural white highlights in the light which suggest silver hair colour, for if it were gray, it would be a dull colour with no sheen especially not like Riku's.