Color variables are definable in Theme -> Variables -> Colors.
Setting color variables lets you enforce a consistent palette across your flow and simplifies the process of changing the colors in your theme in the future.
Creating a color variable is as simple as adding a name and adding the color value. When setting the color, the menu can accept hex values and rgba values. You can also use the palette interface to find a color you like, which will automatically update the hex and rgba values for you.
Color variables are accessible when styling the color of components in your flow. You will see your available colors variables at the bottom of any color menu.