interface Colours {
  red: string;
  green: string;
  blue: string[];
}

let colours: Colours;

export type PropertyNamesOfType<T, TT> = { [K in keyof T]: T[K] extends TT ? K : never }[keyof T] & string;

function getColourStr(colour: PropertyNamesOfType<Colours, string>): string {
  return colours[colour] as string;
}

function getColourArr(colour: PropertyNamesOfType<Colours, string[]>): string[] {
  return colours[colour] as string[];
}

// Success
getColourStr('red');
getColourArr('blue');

// Error
getColourStr('blue');
getColourArr('red');