Plastic molds and silicone molds both work well. I have plastic molds in shapes for use at Christmas I've used for more than a dozen years. The trick I use is to always successfully remove the chocolate candies from the mold is first freezing them for a day. Many come loose of the mold naturally, but inverting over a kitchen towel and tapping lightly on the back of the mold removes the rest.
An ice cube tray might work, but in my opinion, would be boring.
Try hand painting frozen white chocolate with food coloring and tiny watercolor paint brush. Smiles will be your reward.
"Food is our common ground, a universal experience." - James Beard