Someone gave this mug to me years ago as a coloring kit but the markers that came with it were water based (like Crayola) and would not stick to the surface. You think the company would know that. This was a time before brush alcohol markers and liquid alcohol ink. I tried using Sharpies but it left the surface really streaky. Fast forward YEARS later to using Brea Reese Alcohol Ink. Problem solved. I sprayed it with Krylon Clear UV-Resistant Clear Gloss and Krylon Acrylic Spray Paint Crystal Clear and now I have a colorful place to keep my pens.
If you try this, I wouldn't recommend using it as an actual mug to drink out of just because of the chemicals. Even though it does have a varnish, and you can find varnishes that are drink ware/dishwasher safe- it's still better to use these types of things for storage.