The thermometer is essential here and will help you determine when to add the coloring, when to remove the candy from the heat, and when to add the malic or citric acid. If you cook the candy too long, it will begin to caramelize and darken and won't taste right. If you add the acid before the candy cools to 165 degrees F, it will burn and turn bitter. If you add it too late, it may be hard to mix.