I don't know whether teen guys would have used the word aqua very much but teen girls did in the 80s when I was a highschooler and he might have heard the term regularly. I always referred to the eyes of my longtime 1980s boyfriend as aqua colored, for example. Also I remember aqua being the "it" color for girls' tops one school year in the 80s. Coca-Cola made a line of logo T-shirts in the 80s in pastel colors and I had one in aqua. I don't know if teen guys wore aqua prior to pastels for men becoming popular due to Miami Vice in the mid 80s, but my friends and I wore aqua clothes and accessories a lot so Bill probably at least knew what the term meant, which brings me to what I wanted to say about his description of the car color.
It occurs to me that since Bill seemed so afraid of revealing the identity of his tormentors he may have deliberately chosen aqua to describe the car color because the car actually wasn't aqua. He could have just said it was to throw people off the trail of whoever was bothering him because he felt he had to hide their identity. Maybe they threatened someone would come after his family members or friends if he told? Or maybe they threatened to reveal something about him or about someone close to him? Maybe they said they'd kill him the next time they saw him if he said anything to anybody? I don't know but I thought since the word "aqua" might not be a typical word for a teen boy to use, perhaps he chose it on purpose for a particular reason. That reason may have been that he didn't really know anyone with an aqua car and decided to claim the car was aqua to protect himself or someone else. Just a thought, really, but he was so vague about details you'd expect him to be clearer about that it makes one think the vagueness was on purpose due to fear. If so, he may have described the car color as aqua because it was actually nowhere close to aqua.