About 5% of men have some kind of colourblindness. It is common enough I would argue it's good engineering and design to avoid using colour to indicate anything important, particularly when avoidable. Red-green rather than light-dark for intensity maps are particularly infamous. Any time I see one of those posted on a forum and it gets traction, some poor commenter complains at the colour choice because they can't see red-green distinctions. Saw such on HN just the other day.