Crayon Evolution (クレヨン進化論)

Do you use Crayon recently? I don't. I read an article about the history of crayon's color, shown in "Is There an Upper Limit to Crayola’s Law?" Crayola is major company of crayon in the US. The article summarized a spectrum of the colors in a picture. It is beautiful like rainbow, isn't it? This "law" says the number of colors doubles every 28 years.