Use colour to distinguish between lines on a line chart. If the data has an implied order, for example age ranges, use different shades of 1 colour, or colours with different luminance values, but the same hue and saturation.
If the data has no implied order, for example male and female, different colours (hues) can be used.
Lines have a smaller area than bars, which means fewer colours can be distinguished. Try to avoid using more than 4 shades of 1 colour on a line chart without using another mechanism to differentiate between them. Using colour and texture can be particularly powerful.