If you created an account on the website, it will give you access to your purchase history. You will therefore be able to know which products (size and color) you purchased in your previous orders.
An online account also allows you to save your shipping and billing information so you don't have to fill out this info every time. Obviously, your credit card number is never saved for security reasons.
You will also be able to see the status of each of your orders (in processing, refunded, delivered, etc.)
Note that you do not need to have an account or be logged in to order online.