from typing import Optional


def color_mid(value: float) -> Optional[str]:
    """Determine color for efficiency value where "mid" values are the target.

    Args:
        value: percent (0-100) of value to color

    Returns:
        The color string for click or None if color should be unchanged
    """
    if value < 20 or value > 90:  # too close to limit
        return "red"
    if value > 60:  # good
        return "green"
    return None