Seizing
A drop of water turns melted chocolate grainy and stiff almost instantly. Dry bowls, dry spatulas, and no steam from a double boiler reaching the chocolate.
Thinning
If coating is too thick to spread, thin it with a neutral fat made for the purpose, not with milk or water. A small amount changes the viscosity considerably.
Bloom
The grey film on set chocolate is fat or sugar separating, usually after a temperature swing or a spell in a humid fridge. It is harmless and it is ugly, which for a decorated product amounts to the same thing.
Colour
Only oil-based colours belong in chocolate. Water-based gels will seize it, and that is not recoverable for a shell you intend to keep glossy.