You can use any database schema that works for your application. I can't claim to know enough about your business or product line to design the correct database for you with the limited information I have. If the database structure avoids repeated data that can be out of synch and encapsulates all of the data you will need to build your application, then it is good.
Get the dropdown menu populated with your color choices first. Don't worry about how you will tie it to the cart. If you have your list populated so the form looks correct then you are ready to move on to the next step.
Set the values of the list where your choices appear to the ID field in the database for the corresponding row in the table where you will find the new price.
Then you can create a new recordset on top of that page and filter it with the list id that is submitted in the form. That recordset then could return the new price column which you could bind in the add to cart button.