Earlier in the movie, Vivian says
When I was a little girl, my mama used to lock me in the attic when I was bad, which was pretty often.
And I would-- I would pretend I was a princess...trapped in a tower by a wicked queen. And then suddenly this knight... on a white horse with these bright colors flying would come charging up and draw his sword.
And I would wave.
And he would climb up the tower and rescue me.
So when Edward arrives, he's in a white limo (to represent the horse) and carrying an umbrella(?) as a sword, and perhaps the flowers (as bright colors) and then climbs her building (the tower) to resue her from her "prison".
"Why is Edward suddenly in a white limousine?", I suspect that Edward rented specifically to playfully fulfill her childhood fantasy.
BrettFromLA in a comment
This is the most reasonable interpretation. Having realised he's in love with Vivian, he sets out, in the only way he knows how, to fulfill the previously stated dream.