Use a soft box or diffuse your flash using tissue paper or something similar to give you very even lighting across the subject - I assume you'd be working in macro mode? possible the use of a diffuse ringflash will help, but if you don't have any of the above - stick your camera onto manual and use a 150W or higher tungsten bulb (need to custom white balance on the camera) behind a white sheet, get in close camera on tripod and you should be able to pick up skin textures and colour. (colour is a moot point anyway dependent on the camera type - some digital cameras are good with skin tones others are hopeless)