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.