The simplest and most no-fail option to brown trim is to purchase white siding. While you will be assured of a classic match, you may find that a white house shows staining or dirt more easily. In this case, you need to compare ease of selection against ease of care.
The next-simplest option is another brown, lighter siding if you have a dark brown trim and darker for lighter trim. It is not an amazing contrast, but it is also an impeccable match.
Warm colors that insert hues into your house color scheme offer another palette of options. Browns, whether sand, chocolate or in between, have an inherent warmth that harmonizes with warm colors. Muted oranges or orange creams are brassy choices. Deep reds like ruby are less sharp but pack plenty of color. Brick reds, which are more neutral, or a historic red that has a touch or brown mixed in are toned-down warm colors that will please.
Earth Colors and Toned Neutrals
Earth colors from other areas of the palette provide more options. Sage and olive will contrast without clashing. A gray-mauve mixture is understated but rich and has a contemporary edge.