Keys come in different colors. Each color key has a different meaning.
A silver' Might also mean mans greatness and/or goal of striving into a brighter future.
Gold keys represent greatness, size, power, explosiveness, and etc. of the key.
In a Christian perpective, the keys in heaven are the power to enter into eternal life, many Medieval painters, such as Peter Paul Rubens painting of Saint Peter in the 16th Century showed Peter holding tightly to two large keys. This was symbolic of Peter holding close the Bible, or Knowledge of God, close to his heart. Keys, and thus furthering knowledge and learning are an important part of the doctrine of Christianity.