Watercolor can be used on a variety of surfaces, but some are more popular than others. Watercolor paper is the most common surface for watercolor painting, as it's specifically designed to hold and distribute paint evenly. However, other surfaces such as Bristol board, yupo and canvas can also be used.
Each type of surface has its own unique properties that will affect the way the paint behaves. For example, watercolor paper is absorbent, so the paint will dry quickly and won't pool or puddle. Yupo is non-absorbent, so the paint will sit on top of the surface and take longer to dry. This can create interesting effects, but it can also be tricky to control.
Experiment with different surfaces to see what effects you can achieve. There are no hard and fast rules - just have fun and see what you can create!