type RGB = `rgb(${number}, ${number}, ${number})`;
type RGBA = `rgba(${number}, ${number}, ${number}, ${number})`;
type HEX = `#${string}`;
type Color = RGB | RGBA | HEX;
`${…}` notation is available on new (^4.1) ts versions.
https://www.typescriptlang.org/docs/handbook/2/template-literal-types.html