While trim can be any color, including the exact shade of the wall, most people go for a white. What makes that choice interesting is the way white varies, ranging from cool versions with blue undertones to more neutral, pure whites to warmer ones tinted with yellow, pink, rust, or brown. Pure whites—the whitest of whites—are so neutral that they are traditionally relegated to the ceiling.
Color experts generally suggest reinforcing a warm wall color, like peach or marigold, with a warm, creamy white. When walls are a cooler color, like steel gray, turquoise, or blue-green, a cooler white usually feels right. Exceptions work too, of course: Picture the dramatic contrast of cool-white trim with walls painted clean, bright tangerine.
Like all colors, whites change with their surroundings, which explains why paint strips can often be misleading. Rather than compare two whites on the same strip, compare each with the color you've chosen for the walls—first in the store and then by painting a section of wall and adjacent trim. Keep in mind that light plays an important part too. In a room with no sunlight, for example, even a warm white can look a little gray.