In this lesson, we focus on one of the most important channels we have available in visualization, which is color. Color is an extremely powerful channel, and it is ubiquitous in visualization, you can find the use of color in so many different situations and projects. So, what is it used for? There are many purposes for color, but here I want to focus on three main ones. So the first one is to detect patterns in the data. The second one is to label data so that we can distinguish between different categories, and the last one is to highlight specific objects or instances so that you can drive the attention of people into some specific regions or objects. Let me give you examples of each of these purposes. So, the first one is encoding quantities with color so that we can see some patterns in the data. An example is this map. In this map of South America, color is used to visualize information about altitude. So this is an example of using color to encode information about a quantity, and by doing that, we can see a pattern, actually, a geographical or spatial pattern through this specific kind of encoding. This next one is somewhat similar but also different because it is not geographical. Here I have a table, and the frequency of the elements in each cell of this table is encoded with the intensity of the color. Again, this is a way to visualize a quantity through color and to actually make sure that there are some patterns that stand out and hopefully guide you towards interesting pieces of information. The next one is to use color to label data so that we can distinguish between different sets of categories. An example is this scatter plot. In a scatter plot, you can encode information by position but also by giving different color codes to different items so that you can see how they distribute in the space that is generated by these two axes. So, when color is used in this way, it is very useful means to basically figure out where objects of a certain type are located. The final one is to highlight visual elements in a visualization. Here, I have a number of line charts, and in each one, I want to highlight one single line so that you can see this single line in comparison to all the others. So, in this case, as you can see, there is a background that is gray and the line that is highlighted in red so that it stands out. Okay. So these three main uses, these three main purposes; to encode quantities, to visualize patterns, encode categories and label data so that you can recognize where objects of a certain type are, and finally, use color to make some objects standout compared to the rest. So, one thing to keep in mind when using color is that color is very powerful, but it is also incredibly easy to misuse. So, in turn, this means that you have to learn how to use color properly because otherwise, it is very easy to misuse it. Let me give you a first example. This is a map where a quantity is encoded with color and maps to every single county in this map of the United States. Okay. Now, if I ask you, what do you think is the sequence of these values? For instance, is yellow or orange representing a low value or a high value, and is green higher than yellow or lower than yellow, is blue higher than yellow or lower than yellow, and so on. So it is very hard to understand because there is no natural progression of these colors that can be mapped to a quantity. In addition, in this specific example, I have also removed on purpose the legend so that you can not really see the sequence, but let me show you the legend. So, now that you have the legend, you can figure out what the sequence is, but still, the sequence is not necessarily natural. So, it is hard to memorize, and it is hard to use in general. This is an example of color when it is used in a way that is not optimal. As I said, color is very easy to misuse. So, this means, in turn, that what we need to do is to learn how color works in order to use it effectively and that is the main purpose of this lesson and next week's lesson. In particular, in this lesson, I am going to cover two main aspects. The first one is color perception. We need to understand how humans perceive color, how color perception works because this knowledge in turn is going to give us knowledge on how to use color effectively. The second one is color specification. What does it mean? It means that when we are using devices like computers, most likely in our case, we have to know how to specify colors in a way that is useful to visualization design, and not only useful but also natural. As we will see, there are many different ways of specifying color, and I am going to show you why and how certain ways of specifying colors is better than others. Finally, we will focus on color use, on how to use color. Once we have the knowledge of perception and specification, how to use color in visualization design.
