Examine the room. This may seem simple but you want to notice what type of colors you have. If you have brown walls, you can use a lighter or darker brown. If you have a colored wainscoting, again go lighter or darker.
Determine what color the walls are with swatches. This is an easier process if you have kept the original swatch from painting of the walls.
Hold other shades of the same color up to the wall. Select a couple of colors that you may like. get samples of these colors and paint a section of baseboard with the paint. This will give you a larger view than the swatch.