What metal is the bicentennial half dollar made of?

It really isn't possible to say. Silver, when first minted is silver coloured, but after many years and depending on how it is stored it can be nearly any colour of the rainbow. If your coin isn't white silver do not clean it you will only ruin the value of it. Collectors like toned coins, if you clean your coins back to shiny white it will lose value without a doubt do not clean coins.