Epidermal cells consist of a variety of cells including any of the cells making up the skin such as keratinocytes, melanocytes, melanoblast (precursor of melanocyte) Langerhans cells, and Merkel cells. It is important to identify the desired cell type for differentiation. Most methods for keratinocytes and melanocytes are still developing in the literature. That said, there are some preferred methods which have shown success, but would likely require further optimization.
The two references below may be helpful to you:
• Generation of keratinocytes from normal and recessive dystrophic epidermolysis bullosa-induced pluripotent stem cells (PMID: 21555586) - http://www.pnas.org/content/108/21/8797.full
• Functional melanocytes derived from human pluripotent stem cells engraft into pluristratified epidermis (PMID: 21856949) - http://www.pnas.org/content/108/36/14861.long