From my understanding, unless it is a true print defect "boldness" falls under eye appeal subjectivity which is just that: subjective. I think that the issue and the issue's propensity to provide cards with such a characteristic also play a part. Those, however, are just things I think and my history advisor keeps telling me to quit thinking and do more research so I'm probably wrong because I didn't do the proper research.
Personally if the lighter card was not off-center and pulled the same grade or better as the bolder one I'd still pay more for the bold card. Buy the card not the holder.
Edit: Probably should have read your whole post. If the card is missing a color run or had a printing plate issue which should fall under "print defect". PSA has a qualifier for such an occurrence.