Consider the existing décor in your home. Cape Cod houses look very traditional and classic on the outside, but by no means does the interior have to echo that. However, if the paint colors and design style of the downstairs are muted and classic, the upstairs should fit in with that scheme. The same goes for a modern-style downstairs -- take a cue from the bright paint colors and rounded furniture for the upstairs.
Choose a light paint color that goes well with the existing upstairs flooring. A light color will make the room look bigger, just as a deeper color makes it look smaller. If your carpeting or hardwood flooring is a neutral tone, any paint color pairs up well with it. Deeper, more specific colored carpeting must be coordinated with, for example, light green walls look good with dark green carpeting, blue goes well with blue. Dark hardwood floors stand out when paired with a white or light pastel paint color.
Decide to use more than one color in a specific shade. Paint the slanted wall sections one shade of a color and the straighter walls another. This will give the room the feeling of more depth. Or apply the paint with a sponge and layer both shades to create a marbled effect.