No the area that is to be changed is selected such as just the head or hip and a texture map made for just that area is selected. The head textures that just change eye color use the same texture map as the head texture, but only the eyeball iris white etc. is drawn on the map, leaving the rest of the map unused or white. You then select the area you want to change such as iris, white all the eyeball parts and load this texture to change the eye color, leaving the face texture for the rest of the head the same as it was.