This is the #1 problem people have with this recipe. There are 2 possible culprits: overcooking the candy, and using the wrong food coloring. First, overcooking.  The sugar syrup, if allowed to cook to a high temperature, will eventually turn yellow, then amber, then brown. This recipe calls for syrup to be cooked to 290 F, which should produce a near-colorless mixture that will not interfere with food coloring. (It will have a slight yellowish tinge.) If your candy is already a yellow or brown color when you’re adding the food coloring, chances are you have overcooked it, and the yellow color of the candy will mix with the blue dye and turn your candy green.  The solution is to check to make sure your thermometer is accurate (see the Tips above) and to watch the candy like a hawk when it’s cooking. Secondly, using the wrong food coloring.  I have made this several times with Americolor Sky Blue, and have always had the color turn out well. I can’t guarantee other colors or brands will turn out the same. Liquid coloring, especially, can be weaker and can produce a greenish shade. Here are a few colorings other readers have recommended: one reader says that the neon blue McCormick’s liquid food coloring produced a great blue color. She also recommends using a packet of Duncan Hines Frosting Creations in Cotton Candy, stirring it in at the very end and omitting any other flavoring and coloring. Another reader had great success with Adeco brand Electric Blue, and recommends using 5 drops for a great blue color.