What kind of dye I can use to color whipped cream for cake frosting?

You can use some juice and it would add flavor as well. Especially if you reduce it first and makes it more of a syrup. Plus you can reduce the sugar that way

What color do you want? You can color food naturally with items you may already have in your pantry. Here are some examples:

red- hibiscus powder
pink- freeze dried raspberries or strawberries
green- spirulina powder
yellow- saffron or turmeric or achiote/annato seed
violet- blackberry jam

Food coloring? Just go to the baking isle at your local supermarket and look for the multipack of food coloring. Also, there you can find traditional store made frosting, as the whip cream will lose its integrity the more you manipulate it. It just won't hold up as well as frosting intended for cake. Good luck!