White Text on Signs Disappearing After Typing Out

So whenever I type on a signpost using "§f" the white colour option the text will disappear after typing it out for example:

§f-------------

§fSilk Touch 1

§fKnockback 2

§f-------------

The above will disappear after typing it out but if I use the grey colour code §7 instead of §f it works fine. The white code will work sometimes but not always for some reason.