In your current type definition, you have the following:

export type Color = {
    [colorName: string]: string;
};

The above code is the same as saying:

export type Color = Record<string, string>

Instead, you should clearly define each field in the type definition. Defining a Record specifies the key-type and value-type for the object but does not specify the name of each key or the type of each value (in case some keys have another data type).

In this case, you are looking for an interface:

export interface ColorDefinition {
  text: string;
  text_secondary: string;
  heading: string;
  heading_secondary: string;
  background: string;
  background_secondary: string;
  border_color: string;
  primary: string;
  secondary: string;
  muted: string;
  dark: string;
  accent: string;
  yellow: string;
  error: string;
  modes: {
    dark: {
      text: string;
      background: string;
      primary: string;
      secondary: string;
      muted: string;
    };
  };
}

export type { ColorDefinition };

The same way you can also use type to define an interface:

type ColorDefinition = {
    primary: string;
    ...
}

export type { ColorDefinition };

If you would like to write this interface (since you have repeating key-value pairs) correctly, I would write your type definition as such:

interface SimpleColorDefinition {
    text: string;
    background: string;
    primary: string;
    secondary: string;
    muted: string;
}

interface UIColorDefinition extends SimpleColorDefinition {
    text_secondary: string;
    heading: string;
    heading_secondary: string;
    background_secondary: string;
    border_color: string;
    dark: string;
    accent: string;
    yellow: string;
    error: string;
    modes: {
        dark: SimpleColorDefinition;
    };
}

If you want the modes key to be an object with any value for key names, then you can do this instead:

modes: Record<string, SimpleColorDefinition>