The beauty of using CIELAB to visualize color palettes is that colors that are close together are similar, and those that are far apart are different. These are 3D differences; to match, two colors would need to be in the same position on the a*, b* plane and on the L* scale.
In the a*, b* plane, chroma is the distance from the center (gray has a chroma=0). If you draw an imaginary line from the center to the color, the hue corresponds to the angle, as shown in the figure. Hue=0 is pink, green is hue=130, blue=270, etc.
Defining hue angle on the a*, b* plane.
Overall, categorical palettes tend to contain several distinct hues and a range of lightness values. A full-hue circle like the Tableau 10 is a good default because the colors are visually very distinct and correspond to simple color names like red, blue, purple, and brown. To create the Tableau 20, we use light-dark pairs of these basic colors. Below we can see that Tableau 10 contains 10 different hues, with varying chroma. The gray has a light tint. Our classic Tableau 10 has a similar structure, but the colors are higher chroma and somewhat darker.
The top row shows new and old Tableau 10 palettes, and the bottom row shows new and old Tableau 20 palettes.
Palettes with fewer hues have a more distinctive visual style, which can be considered more aesthetic.
The left is our winter palette, and the right our summer palette.
In contrast, the palettes we use for quantity trace a continuous path stepping uniformly from light to dark in a small section of hue space.
Here is the Orange palette, which varies from a light orange-tan to a dark brown. Note that the hue for brown is the same as for orange, but it appears brown because it is dark and lower chroma. In fact, any yellow, orange, and red colors can be made to appear brown or tan by manipulating their lightness and chroma. Diverging palettes trace two paths through a neutral center.
Note that these paths are not straight lines. The variation in hue and chroma that makes them twist and curl also makes them more visually effective.