Using water, nail polish, and a white ceramic mug, you can easily create your own artsy DIY coffee mug with a "watercolor" effect colored on its surface.
Simply fill an empty, disposable container (like a disposable pie pan) with warm water and add a drizzle of nail polish. You can choose to go monochromatic or combine multiple colors.
Use a toothpick or wooden skewer to stir up the colors and then holding the ceramic mug by its handle, roll the mug into the colored surface.
If you don't like how the colors look or if the colors get into the inner surface of the mug, you can always use nail polish remover to take the colors off. Once you are satisfied with how your mug looks, you can let it dry for a few hours or bake it in the oven.
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