Don't be afraid to paint your walls a deeper, darker hue. There's a misconception that small rooms must be painted white or a very light neutral. Of course, if you like white walls, go for it! But, don't be afraid to experiment. Darker colors can actually create an illusion of depth in a room and bring some drama and interest.
If you are using a darker paint, keep in mind that warm colors (yellows, reds and oranges, etc) advance, while cool colors (grays, greens, blues, etc0 recede. To open up a space, try paint colors with cool undertones.
Ultimately though, what matters most is that you like the color palette of your room.