[qualiteam]

You should decorate the \XLite\Controller\Customer\Cart::processAddItemSuc cess() method from your custom module and make it do nothing (check how Add2CartPopup module does the same thing) - this will disable the "product has been added" top message completely.

However, there are other messages that may appear as a green top message. You can override the CSS rules for "#status-messages .info" selector and change the colours to yours from either a custom module, or by using Theme Tweaker module (Look & Feel -> Custom CSS).