If there is a high oil content in the candy, pens alone won't work well. It might be that you need a piece of edible paper to decorate and (easily) apply to your mints as a kind of edible sticker or label. Our edible paper comes in white and a large selection of vivid and pastel colors as well as our metallic gold, silver, pearl and bronze which we call our Dazzle colors. They also come in pre-cut circles and card shapes.
Another option that works very well is using chocolate transfer sheets (which can be used for non-chocolate candy as well).To use transfer sheets simply pour the warm sugar based candy such as tempered chocolate, caramel, or any soft crack stage sugar based candy, over the chocolate transfer sheets. When it hardens you will see the design as you peel off the transfer sheet. We carry both pre-printed transfer sheets and blank transfer sheets which can be printed with edible ink.