What light does is photodegradation: a molecule is broken down by light energy. The colour often stays the same while the active substance falls and degradation products rise. The EU Good Distribution Practice guidelines require medicines to be protected from the harmful effects of light (point 5.5) and list light among storage conditions (point 3.2.1).
The outer carton is part of that protection. A vial taken out of its carton to save space during picking is unprotected from that moment; amber glass helps, but the product's own instruction governs. ICH Q1B runs light testing in steps from the exposed product to the immediate pack and the marketing pack, so the protection a carton gives is something established by testing.
Exposure accumulates: half an hour in the picking area, ten minutes on the ramp, two hours in the pharmacy each look minor alone, but the total can pass the product's limit. A clear bag on the last mile, a glass-door cabinet and a ramp in the sun are the most common points. Carry the product in its carton and in an opaque outer, set aside a shaded waiting area at the ramp, and give this product group its own line in the warehouse instruction.