One maybe enough for some people but for me, it isn't. Well, I'm taking about the highlighters I use when reading my books in medicine. I guess I just need to see more colors when reviewing because it stimulates the brain to fire signals and helps in memory retention. See this yellow highlighter of mine. As I was reading my Obstetrics book, I noticed that a hint of orange at the tip. The over lapping of the orange and yellow highlighter might have caused this. But anyway, it's actually fascinating to look at.