Choose a concealer that is on the heavy side and is more cream than liquid based. Be sure to choose a product that matches your skin tone, otherwise it will not properly conceal the cold sore. For cold sores that are more on the red side, consider applying a green or yellow-based concealer first before applying skin tone based concealer. The yellow and green in the concealer will help to tone down any redness. Apply a finishing powder to set the product and ensure it stays on. Always remember that since cold sores are contagious, you should discard the makeup sponge after application.