Even when we just look around, a wide variety of colors leap into our eyes. We are surrounded by an infinite variety of colors in our daily lives. However, unlike length or weight, there is no physical scale for measuring color, making it unlikely that everyone will answer in the same way when asked what a certain color is. For example, if we say "blue ocean" or "blue sky" to people, each individual will imagine different blue colors, because their color sensitivity and past experiences will be different. This is the problem with color. So let's study a little and determine what kind of color information would be useful.