How can we retrieve the evolution of the state of an smart contract variable?

I have a simple smart contract with a variable color which stores a string. If I unterstand correctly, each transaction can change the state of this variable and is stored permanently into the blockchain. Let's say tx#1 changes it to red, tx#1 changes it to blue, and tx#2 to red, tx#3 to green. I want to have red, blue and green, and not just the last state of this variable color. How can I achiev