```python
def hex_split(colour: str) -> list:
    if len(colour) != 7 or colour[0] != '#' or not all(c in '0123456789abcdefABCDEF' for c in colour[1:]):
        raise ValueError("Invalid colour format")
    return [colour[i:i+2] for i in range(1, 7, 2)]

def hex_join(parts: list) -> str:
    if len(parts) != 3 or not all(len(part) == 2 and part.isdigit() for part in parts):
        raise ValueError("Invalid parts format")
    return '#' + ''.join(parts).lower()
```