```ts
export function hexSplit(colour: string): string[] {
  if (!/^[a-f0-9]{6}$/i.test(colour)) {
    throw new Error('Invalid colour format');
  }
  return colour.match(/.{2}/gi) as string[];
}

export function hexJoin(parts: string[]): string {
  const hexDigits = '0123456789abcdef';
  return parts.join('');
}
```