You may have possibly misunderstood the answers herein.
Red pattern lines are against a white background, not black.. When possible, if the cut areas are filled in with light grey, it makes it easier to distinquish the area to be removed. Often this is most important on busy, multi holed, patterns.
Many patterns that are available in a photo file format (JPG or PNG) or PDF's are provided in Black lines, or in some cases totaly black areas that are to be cut. A program like Inkscape can be used to convert these black line patterns into scaleable patterns with the linework in red.
That being said, not evereyone but many of us can see blade against the red ( I use a pinkish red) lines better than black lines.