Το λευκό φύλλο;
Why not Η άσπρη σεντόνι?
adjective always before substantive, as in English?
Yes! Let's say that... :D
There are exceptions when you could move an adjective after a noun but they all have to do with speech changes that show emphasis or more literary constructions, so you shouldn't worry about them at all.
Could this be a white sheet of paper?