Fire hydrant

I am filled with water, yet I remain empty. I move around my object, when the sun changes direction. My object may change colors but I stay the same. There is never two of my objects next to each other. I am a fire hydrant's shadow. I am unable to hide, but I am hardly noticed. My object has purpose, I do not. My form changes when my objects does, unless it's color. I can't rust unlike my object. We don't talk to each other, for we both don't have a mouth. We don't walk because we have no feet. I see nothing but darkness, but I am not afraid because I have no emotions and neither does my object. I don't fret about my life anymore, nor do I worry about disappearing, for I can't ever really disappear. My object can never truly disappear, so I am everlasting.