The course mentioned paying attention to red/green, because that is the combination that is affected by genetic color-blindness, but if you're using a lot of colors where it is important to know which is which, consider making sure that there is no one else with a different problem combination. I have a friend who cannot see the difference between anything from light orange to dark red - head injuries can affect color perception and today we have lots of veterans entering classrooms after experience traumatic brain injuries. Or consider, in addition to using different colors, using different types of lines (dotted, dashed, dot-dash-dot, etc).