I would always say 'as blue as THE sky'
You need an article before "sky" in this instance.
In Finnish, can we simply attach a noun to a colour, as in "sky blue," "robin's egg blue," "cherry red," ""blood red," or - - everybody's favourite - - "sh* * brown."?
Actually, "taivaansininen" sounds better than "sininen kuin taivas".
Yeah, that's how you'd usually express this (and the others too).