Nước màu translates to "colour water", which is typically made with caramelizing sugar until you have a deep amber colour. A lot of people make this at home and don't use a concentrated syrup, which would mean you use about 1 tbsp for this dish, but I'm using the concentrated syrup bought at the Asian market. If you don't have access to this but you have molasses, use molasses. They have a similar flavour profile and will add colour to your caramelized meatballs!