If you mean to model the tiles there are several library parts and add-ons available. (My good friend Til Breton has one available
here.)
To make them white should be just a matter of applying the correct material but the possibilities (and method) may vary depending on how they are modeled.
If you just want to texture map them you can edit/create the image file in Photoshop (or GIMP, or whatever) and assign this to the roof material.
