You need to have a column in your cart object to for the color option. you will likely want to set the column to be unique.
In the add to cart button, select the new color cart column, and set it to be update able from a hidden form field.
this will add a hidden form field to the add to cart button form, set the value of the hidden form field to come from the recordset.
When you create the cart display page, make sure you select the color column on the second step of the display wizard so it will show when the item is added to the cart.