You need to use candy colors which are oil based and it should work just fine. I have never had any problems tinting chocolate. Just don't use regular icing colors because they are water based and will make the chocolate/bark seize and you will not be able to pour the chocolate.
You should be able to find candy colors at both Hobby Lobby and Michaels.
If you can't find any you can use shortening with regular icing colors. The shortening will keep the chocolate from siezing even with the addition of water. It's also something to remember if you ever get a little water in your candy.
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