How can I add flavorings and colorings to white chocolate?
Have you checked your food coloring to make sure it isn't water-based? The addition of any water into chocolate will cause it to seize. You might be better off looking for a powder like this one: https://www.ckproducts.com/categories/276/Powder-Color-Cake-n-Candy
Because it's a powder, it won't cause any harmful side effects to your chocolate. They sell it at most cake art stores, and potentially at places like JoAnn.