On the bindings panel of the Add to Cart Button Server behavior, the color and Size columns should be set to be updateable from a Select List, and the default value should not be bound to anything.
You have created the select lists yourself and set the default value for the Color and Size to the recordset columns.
remove the select lists you have created from the page, Then on the bindings panel of the Add to Cart Button Server behavior, set the color and Size columns to be updateable from a Select List, and the default value to nothing.
This will cause eCart to create Select lists on the page for you that you can populate with the values from your recordset, the selected value in the list will be sent to the cart.