Answer:
I mirror your actions.
I am a king who's good at measuring stuff.
I am pronounced as one letter written with three. I come in blue black brown or grey. Reverse me and I read the same either way.
I saw the dead bring forth the living. I saw the living bring forth the dead. What did I see?
I make you weak at the worst of all times. I keep you safe I keep you fine. I make your hands sweat and your heart grow cold. I visit the weak but seldom the bold.
Whoever made me don't want me. Whoever bought me don't need me. Whoever use me don't know me.