Just an FYI, I made "eyeballs" for Halloween and didn't have red velvet cake so I used white and put in red food coloring. Umm..lots of drops later I still had a pink cake. Now, I am by no means a professional , but I have no idea how to make a white cake red. I used almost a whole bottle. Maybe my red was bad? To be safe, I'd use the red velvet.
But more importantly....Your balls look amazing! I was going to do some red and green for the holidays.
Food colouring (assuming you mean the little bottles of liquid) isn't strong enough. We use either a paste or power version that we get a baking supply shop nearby. Works for cake batter and icing (so you get a true red, for example, rather than a pastel red).
Store them at room temperature. If you chill them, the chocolate could sweat or crack as it comes back to room temp.
I'm adding these to my baking list this year. Thanks!