Semantic Theming - indexed palette

There are many articles on palette categorizations online, I would skip to semantic relation of palettes to UX elements. 

Usually palette is selected from basic branded colors base. Usually 2 or 3 is sufficient, perhaps it relates to the heraldic symbols and colors. Remaining colors are chosen with keeping the base ones and emotional palette in mind. 

There is a catch on selection enterprise and government designs are facing: color set should be "accessible" for various cohorts of people: color blind, light sensitive, etc. Adobe has given a nice tool to help with accessible palette creation. In regards to color "accessible" means that close colors should be distinctive enough even for people with color challenges or in applicable conditions of low/bright light.

The palettes would have various colors but some of them have to be in straight color transition like bright to dim. Why you would need a color sequence? The answer resides in various states UX component are presented during interaction with UI. Those states are used in sequence: normal follows hovered, which followed "active"(pressed) , the disables should be least noticeable and resides in the end of color sequence. See Buttons and Actions states. 

How many color sequences you would need? It depends on how many color variations your application supports: light, dark, high contrast for each, grayscale for ePaper devices, etc. Another factor is munber of variations for UX components. As in Buttons and Actions there are 5 variations proposed, will be more for other types of UX components. Each need to have own entry in one of indexed palettes. 

While same palette could be reused for different UX components variations, it is better to choose a significantly distinctive different entry point in palette for inside of same color theme variation. 

For UX components single palette is not sufficient. The indexed palettes usually come in more than one color per row. You would need to make colors as for background as foreground/text, for outline, and for other sub-elements. For action it would be 3 colors: background, text, outline/border. The 4rth is assumed to come from theme variation background which is a basis for selecting proper entry within indexed palette. In this sample you could see the need to alter the text color due to background change:

Now to the meat of subject. The indexed palette become semantic when one of its entries is associated with one of semantic UX element variation.