Complex sentence: Officers wore a short or long sleeved lightweight white (or off-white) tropical shirt with the green trousers and when they wore the green tropical tunic they usually wore the shirt collar outside and over the tunic collar.
Split into simpler sentences: Officers wore a short or long sleeved lightweight white (or off-white) tropical shirt with the green trousers. When they wore the green tropical tunic they usually wore the shirt collar outside and over the tunic collar.