If the 'Add To Cart' button is not turning GREEN, it indicates that at least one customization option is missing.
 
To see which selections are missing, first click the builder menu (three horizontal lines in the top right over Personalize) and then toggle on the 'Only Show Incomplete' option. 
 
This will show you what needs to be completed before adding your item to the cart. 

STEP 1

STEP 2