There isn't a way to fill it because it isn't a constant colour, if that makes sense.
Like, if it was all the same shade red you could do it, but because it isn't you can't, sorry.
You can though, use the pippet tool to get the colour of the skin near the head you put on and use the brush tool and paint on the colour slightly and then do the same with the head so they merge and there isn't an obvious join.
I don't know if that makes any sense to you![]()
You make it sound as if it can't be done, which simply is not true. Now, mind you, I know very little about photoshop, and could not do it myself, but I am sure that someone experienced could do it seamlessly.