Generally, the more pattern in a room, the greater the sense of coziness. Think of deep rugs, or your fur throw. Soft carpets and fabrics convey a sense of warmth, intimacy, and privacy. You could also get away with a room-sized rug, but avoid area rugs. Pale neutral or cool colors.
Pale colors reflect light and make the place seem airier.
Neutral and cool colors recede, they seem farther away than they actually are. They break up space, and you want to smooth out the space. Make sure the surface is even, not shaggy or carved. Repeat indoors the colors you see outdoors. If you see green grass all year long, pick a deep-pile carpet in the same green and then the indoor and the outdoor spaces will seem like one continuous lawn.