To describe what a person is wearing, the prepositional or accusative case is used?

Answer

Hi! You can use both Assusative and Prepositional, it depends on the construction you use.

For example,

Он в тёмном пальто и белом шарфе.
He is wearing a dark coat and a white scarf (Prepositional case is used).

Он одет в тёмное пальто и белый шарф.
He is wearing a dark coat and a white scarf (Accusative case is used).

These constructions have the same meaning:

1. Я (ты, он, она, мы, вы, они)  в + Prepositional case

2. Я (ты, он, она, мы, вы, они) одет(а, ы) в + Accusative case.