Why is Gandalf Called Gandalf the Grey??

[dancing spawn of ungoliant]

The colours need not stand for anything in specific.

But why Gandalf after his return then is white and he kind of makes a big deal of it? Or is it just one way to say clearly that Gandalf has taken the place of Saruman as the head of the White Council.
And what's the thing with Sarumans 'new dress that changes colour'? Whoooops, this off topic...sorry, but I'm just curious.