"If we offer a discount of 10% for 10+ purchases of an item, and someone has ordered 5 blue and 5 red, then will eCart pick up on that and still apply the discount?"
depends how you set up the discount.
if it is set on a product ID, or category ID, it will apply regardless of the options added
see these tutorial for details on product options:
http://www.webassist.com/tutorials/Add-dynamic-product-options-I-eCommerce-Series
http://www.webassist.com/tutorials/Add-dynamic-product-options-II-eCommerce-Series