The red colour in sweet and sour pork typically comes from the addition of red food colouring or a combination of red ingredients used in the sauce.   While there can be variations in recipes, the vibrant red hue is often achieved by incorporating ingredients such as ketchup, tomato sauce, or red chili sauce. I don't use red food colouring in mine, but the red colour is still quite vibrant because of the addition of tomato sauce in the sweet & sour sauce recipe.   In some cases, red food colouring is used to intensify and enhance the red colour of the dish. Food colouring is a common additive used in cooking and baking to alter the appearance of dishes. If you want to have a very vibrant sweet & sour pork, feel free to add a drop or two of the food colouring.