Here's an improved way to use magnets to hold your water cup to your metal watercolor set. I embedded three small neodymium magnets (1/4"x1/16") in an epoxy sculpting compound called Magic Sculpt. The compound sticks to the bottom of the water cup.
This way I don't need to drop a magnet into the water itself, which will rust eventually. And it holds better to the metal. It would even hold just fine if you held it upside down and shook it.
Neodymium Magnets 1/4" X 1/16"