Now, after learning a little bit about how color is perceived by human vision, the next step is to understand how to specify color. What do I mean, specify color? Well, I mean that when we want to practically use color, especially in visualization as we will see later on, we want to be able to say, I want this type of color. Or I want these colors to have certain properties. So how do we specify colors in a way that is helpful for visual representation? We need color specification systems. And we need color specification systems that are reliable and uniform across different devices. If you think about it, that's not a trivial problem at all. So if I want to reproduce a specific kind of color, I want to be able to use a specification that can be reproduced in different devices, which is a quite complex problem. So, one important thing to know, as I already mentioned earlier, is that every color can be expressed as the sum of three colors, or in general, as we will see, every color can be expressed in a three dimensional space. So let's start with a somewhat simple example. If you have three lights and one light is blue, one light is green, and one light is red, what we can do is to change the intensity of these lights to basically match an existing color. If we can do that, we actually have a way to specify numbers that describe a specific color. Think about having a way to create a standard set of three lamps, one is blue, one is green, and one is red. And let's assume that it's easy to reproduce a standard version of these lamps. Then if I want you to reproduce a given color that I've tested earlier, what I have to do is just to give you the number or the value of these three intensities, the intensity of blue, the uncertain intensity of green, and the intensity of red. So this is the basis of a very large set of color spaces. What is a color space? A color space is a system typically of three variables, in a three-dimensional space, that describes uniquely a color. By giving, I give you three values, I'll tell you which space I'm using, and you'll be able to reproduce exactly the color that I want to communicate to you. So that's a very important concept and a very useful one in practice. One thing to know about color spaces, before we dive into specific color spaces, is that the three primary axis of each color space basically define what is the gamut of the color space. We choose the whole set of colors that can be reproduced by this specific color space. One important thing to know is that not all color spaces are equivalent. What do I mean by that? I mean that a given color space may reproduce a subset of the full set of colors that humans can naturally perceive. For instance, the RGB space, which I'm going to introduce next, can cover a subset of all the possible colors that human can perceive. In the next lesson, I'm going to walk you through different color spaces. And as we will see, these color spaces have different properties that make them more or less useful for this organization design and color specification in general.