Can you paint metal with watercolor?

No! It would just flake off like chalk after drying!

Gouache is a watercolor-like paint made with gum arabic and chalk (which makes the color opaque, i.e. you cannot see through the color). When dry, it has a unique, velvety texture and rich, deep color. Years after a gouache painting has dried, it can still be blended and re-worked with a wet brush. It was the standard medium in fashion illustration and popular with illustrators until the introduction of commercial design markers.

Use it on illustration board!