UI Toolkit or uGUI. Whichever your project needs.

// Layout in UXML, look in USS. The ghost trails through a USS transition:
//   .health-bar__ghost--trailing { transition: width 0.6s ease-out 0.45s; }
[UxmlElement]
public partial class HealthBar : VisualElement
{
    readonly VisualElement fill = new() { name = "fill" };
    readonly VisualElement ghost = new() { name = "ghost" };
    float target = 1f;

    public HealthBar()
    {
        AddToClassList("health-bar");
        Add(ghost);
        Add(fill);
    }

    public void SetHealth(float pct)
    {
        pct = Mathf.Clamp01(pct);
        bool damage = pct < target;
        target = pct;

        fill.style.width = Length.Percent(target * 100f);
        ghost.EnableInClassList("health-bar__ghost--trailing", damage);
        ghost.style.width = Length.Percent(target * 100f);

        if (!damage) return;
        AddToClassList("is-hit");
        schedule.Execute(() => RemoveFromClassList("is-hit")).StartingIn(120);
    }
}