Heya! Thanks! and sorry for the late reply, been busy af.
the blue and red leads are just lighter and less likely to show up when I scan something in, in black and white. They are softer lead and take a little getting used to so I just used them all the time, even on preliminary sketches. a side benefit is I can rough sketch with blue and then refine it a bit or make changes in red and it's easy to see what is what....if I used one color for the rough and refined/changes, it'd be a big grey mess.