Color combinations should look good together, but should also be easy to differentiate. Using value (the lightness or darkness of a color) in addition to hue can help make your colors more distinct from each other.
It is easier to perceive approximate differences in length than it is to perceive differences in color—say, when something is twice as long versus twice as light. As such, when using colors for categories it is best to group them into visually distinct bins.
Using distinct color values can also help make the chart legible to persons with color blindness. Some software packages include color palettes designed with color blindness in mind and these are helpful to look for.