You can mix them with no problems. Even better that it's the same brand. Start adding just a little of the white to the blue at a time. You will be surprised how a little bit of white will lighten up the blue. Just make sure you mix it well. When you think you have mixed it enough, you are about 1/3rd of the way there. Remember, you aren't just mixing two colors, you are mixing two sheens (Shines)