How Can I Make Flour More White When Under Process In Flour Mill
Flour will whiten over time when stored. It generally takes several months to get a whiter color. Most flour manufacturers wish to speed this process up and so they actually bleach the flour using cholrine or benzoyl peroxide. This bleaching process also removes nutrietns which is why flour in the U.S. that has been bleached must also be "fortified" by having nutrients added back in.

There isn't much you can do beforehand. After milling, you can either bleach for immediate affect or store and allow it to whiten naturally. Keep in mind that different flours are usually bleached with different chemicals depending on what you plan to do with the flour.

I'll leave it to you to decide if it's worth the time and nutritional value to mix chemicals in to your flour for what is purely a cosmetic benefit.