If your house is two or three stories tall and you want more than one shade of the principal body color, it is tasteful to apply the darkest shade on the first floor, medium on the second floor, and the lightest on the third floor. If the dark color is on top, you risk making the house appear top-heavy. Houses with shingled upper stories are the exception; these should be painted a lighter shade on the lower story.
Gutters and downspouts should be painted to make them as inconspicuous as possible. On a frame house with olive trim and a light green body, for example, the gutters would probably be olive to disappear against the olive cornice, but the downspouts would be painted light green to correspond to the adjacent siding. On masonry buildings, the downspouts often are painted bronze green to simulate weathered copper.
In climates with heavy rainfall or snow, dark-color roofs will be more stain resistant so you'll want to choose other colors that work with gray or slate colors.