I have diabetes. And among other damage it did, was on each shin a curious blotch appeared. One leg had it much sooner than the other. A leg injury some 25 years ago probably sped it up. Doesn't hurt or impair walking or running. Wikipedia said the mechanism is not understood. Common among diabetics.
If you're just using markers, (Besides Sharpie/Permanent markers) it usually just washes right off. If your skin is dry or you have makeup on (Like if you're doing it on your face) I suggest being a little more cautious- but even if it stains a little, it won't be very bad at all. The food coloring-based bloods usually leave a light pink tint on skin, hair, and clothing.