There are several finish choices to select from when designing a landscape lighting project. Generally, the best rule of thumb is to select a color that allows it to disappear into the landscape during the day because once the sun sets all we’re interested in is how the light emitting from the fixture looks on what it’s illuminating. After dark, you won’t see the fixture itself in most cases.
In the Midwest, the most popular finish color is dark brown primarily because it blends into the landscape so well. Painted (powder-coated) aluminum is generally the least expensive, but there are competitively priced brass fixtures, too. Painted fixtures can be compromised if salt spray or harsh chemicals come into prolonged contact with the fixture housing causing paint to chip, peel or blister. Brass has no painted finish and it withstands the elements, chemicals, etc. better than painted aluminum.
Another option is stainless steel fixture housings which tend to be popular with more contemporary looking architecture. They also blend in well with lighter colored / painted decks and structures as well as natural stone.
Some homeowners will select black fixtures because they match with black fixtures on the house, but there is no rule dictating that requirement. Usually the goal with landscape lighting is to hide fixtures in the landscape so you don’t really see them during the day, and you only see the effect of the lighting (not the fixture) at night.