Okay. An interesting variation over the single hue sequential color scale is a more multi-hue sequential color scale. So, what is the idea here? The idea is to still have a perceptual sequence of values that changes according to lightness or luminance, but go through multiple hues. In the previous example, I've shown you, I kept hue constant. In this one, in this new method, now, I'm trying to do exactly the same thing, create a perceptually linear color scale that changes color intensity or lightness uniformly, but goes through several color hues. Let's see how this is done. Once again, I'll give you a demo. I'm using the same color picker. But, what I'm doing right now, so, in the previous example, what I did was to position the picker vertically parallel to the lightness axis. Sorry. But that's not the only way to create a perceptually uniform color scale. Let me show you what we can do. Well, we can start from, since on the x-axis we have hue, we can basically horizontally go from, try to span as much as possible of the hue space, but at the same time, also go through colors that are very dark and very bright, okay? As you can see on the right, what we have now is a color scale that increase linearly and uniformly in terms of lightness, which is the parameter we are mostly interested in when we want to convey a quantity, but at the same time, it's changing color hue, and as we will see in a moment, that's a useful property. I can do exactly the same thing by flipping the sampler, the color picker, in this other direction. So, you see we go from very dark to very bright, but we also span a lot of different colors and lots of different color hues. This is even more evident when I reduce the number of samples. Look at this. So, we go from very dark to very bright, but we also cover different color hues. So, now, why use a multi-hue sequential color scale? If we are happy with a single hue, why should we use multiple hues? Well, there are a number of important reasons. The first one may be just aesthetics. It's possible sometimes that using multi-hue color scales, the result is more aesthetically pleasing, but that's not the only point. Another one is that by going through different color hues, we span a larger volume of the color space, and by doing that, we go to a higher set, a larger set of colors. In turn, this means that we can create a higher, a larger set of discriminable colors. Colors that can be discriminated among them. So, in a way, we have higher resolution. When we allow ourselves to span multiple hues in the HCL color space, now we have a larger space, and because of that, we have a higher resolution, which is important. Finally, and probably even more important, we have that by going through multiple hues, we can communicate two pieces of information at once. We can communicate information about the quantity that is mapped to color, but we can also segment areas that are mapped through color into areas that can be very easily labeled. I can say, so this is the red color, this is the red area, this is the yellow area, this is the blue area, for instance. Let me show you an example that makes this more clear. Here is an image that is being generated with a multi-hue perceptually uniform color scale to represent some astronomical data, okay? As you can see here, there are different regions, each regions represent areas where the quantity is similar, but since we are going through multiple color hues, it's also very nicely segmenting these areas into regions that are easy to label and even name. I can talk about the red region, the white-blue region, and the yellow region, and the dark-black region. So, I can actually name them. This is not easy to do with color scales that use only one color hue. Let me give you another example. This is another quantity map mapped in the UK with three different uniform color maps that have the same property. They span different color hues, but they're also perceptually uniform in terms of lightness. As you can see, not only information about quantity is well-represented, but also information about regions. I can segment regions according to different hues.
