In this case what you need to do i added a column to eCart and call it discountedItem. The value for that will only be set to 1 if it is a paint item (You will need to add the bindings to the add to cart button). Then in eCart, for the discount url, you will need to use the Condition 'number of items with a specific column value' which has a value of 1 is > 0.
let me know if you still have an issue.