I'm guessing the "no red ink" dictum has its genesis from indelibility concerns. Written records need to be made with indelible ink (not erasable, not easily smeared or smudged, no pencil). I remember years ago that was a legit concern with many commonly used red pens - for whatever reason red ink was more susceptible to smearing (for example, the document got wet and it was wiped dry). However, I just did a highly scientific test i.e. made a red, blue, and black scribble; let the dry 15 seconds, licked my finger and tried to smudge the marks. No smudging/smearing with any. So, my 2-cents is if the red ink (or any color for that matter) is indelible then its good to go.
