Neutral colors are a good start, as they are both calming as well as easy to match decor with. Think beige, grey, and even muted or earth tones. Another key to coziness is lighting. Harsh, overhead lighting might be the last thing you want when you flip on the switch at the end of a long day. Try instead to incorporate lamps, twinkle lights, and even candles so that you can wind down with ambient lighting.