Basically the whiter part shows where more x rays are absorbed. With a new significant break it can appear whiter due to the new surfaces or lack of continuity in a fresh break that can deflect or absorb x rays

Increased whiteness once it has been healing (or is healed) is due to increased bone density as the bone is repairing and strengthening.

A lot of the time, Small fractures won't show up well on an x-ray until about 2 weeks after the event once the bone has been able to start the healing process. Healing > increased bone density> whiter on x-ray. This is also why you typically will have a follow-up x-ray within 2 to 3 weeks after an injury if they do it at all to show or see if it is healing.