After lots of experimenting, I finally figured this out and as you can see, it works. I decided to go on to make my second character. I repeated this entire process, but the face doesn't work for some reason.
These two characters are exactly the same except for the hairstyle and the colour of the body and clothes. I have checked my work multiple times and have been unable to notice any discrepancies between the two. The only difference I have noticed is that in the node editor for the second character (the one that isn't working) the nodes snap to the grip, while the nodes for the first character do not. I am not sure if this makes any difference, but it is just something I have noticed. The last thing is that for the second character, the colour I set in the RGBMix node renders, but the face does not.
I would really appreciate any advice anyone has regarding this problem. Please help!