Surgeons mark on skin. They need a marker that is completely non-hazardous, won't accidentally rub off, and is convenient to store and use. Felt tip markers in the retail market use ingredients that, whether disclosed or not, shouldn't be applied to skin. A crayon or grease pencil is your best bet since there is not irritating solvent content.
The Mistubishi® Dermatograph, like the name suggests, is perfect for marking on skin. it is a high quality grease pencil. Some colors such as red are more visible than others but all are removable with rubbing alcohols or regular soap.