Was he blind from birth? if so, he couldn't comprehend what navy blue was - it's just a name after all and doesn't describe the colour at all. In fact, we all perceive colour slightly differently due to the fact we're individuals and certain variables mean it's impossible for us all to see exactly the same colour. In my years of working as a graphic designer I've had so many conversations about the subject:

Customer: "I'd like my business cards printed pillar box red"

Me: "Did you have any particular pillar box in mind sir?