def compare_labels(a, b):
    """
    Compares two GitHub labels (name, description, color) and returns a list
    of fields that are different. Returns empty list if there are no differences.
    """
    diff = []
    if a.name != b.name:
        diff.append('name')
    if a.color != b.color:
        diff.append('color')
    if a.description != b.description:
        diff.append('description')

    return diff