The poll isn't even necessary...this isn't something that is really debatable. The Aliens are brown, regardless of people's perceptions to the contrary.
As SM noted, Khondji processed the film using the silver-retention process, which makes the blacks much deeper and richer. Consequently, any dark colours become even darker, making it hard to tell where they end and the shadows start.
The cinematography is easily one of the best aspects to A:R.
Or, in Carter-Speak: The film was shot to look dark, so the brown costumes look almost black.