Color plays a big role in determining the mood in a room. As a
guide, allow your main color to occupy 75% of a room (including
floor and wall space), with 15 to 20% for your secondary color
and 10% for an accent color. To visually expand a room, keep
the color contrast between wall and window low. To make a room
feel cozier, create a high contrast.

As a general rule, use neutral- and pastel-colored window
treatments when you want other furnishings—a prized painting,
say, or sofa—to make bold statements. Also, neutral and pastels
colors not only invoke a sense of calm but create an illusion of
height. Conversely, bold colors play down a room's loft; select
bold colors for your window treatments if you want them to be the
room's coordinating feature or its chief attraction.