Unfortunately, this may be a condition that is more related to thermal changes, that to sunlight, so there is also a good chance that wide spectrum lighting wouldn't help. There are no scientific studies that I am aware of that support this notion but it seems reasonably safe to try Vitamin E. Phemphigus foliaceus is an immune mediated disorder that German shepherds are somewhat prone to. It usually causes inflamed skin or small sores, along with depigmentation. It doesn't sound like these lesions are present but it would still be worth looking carefully. Eyelid margins, interdigital spaces and ears are also commonly affected when this condition is present.
Hypothyroidism is reported to cause depigmentation occasionally.
I think that this is a pretty uncommon problem to surface as the sole sign of hypothyroidism but that is the reason that kelp is reported to be helpful in a small number of dogs with nasal depigmentation. Kelp is high in iodine and may help with subtle hypothyroid conditions. It is extremely easy to misdiagnose hypothyroidism, so caution should be used in searching for this condition based on limited clinical signs. I know this wasn't much help, really. Your local vets are probably not disinterested.