When creating a new palette (ordinary or global), the opacity (alpha channel) is copied from the selected object (if there is one). There is no way to change the opacity after palette colour creation. Even using "Edit Fill..." and using the colour picker does not work.
So if (for example) a global palette colour is created, accidentally with a opacity value of less than 100%, it cannot be later fixed. It would require a new palette colour to be created, and each object referencing the incorrect colour to be modified to use the corrected colour. Thus in part defeating the purpose of global palette colours.
Thanks retrograde. I was out for most of today, and I think stepping away from it has helped. Yes, I think building up the shadows beneath the nose will work, and I realised too that I had accidently removed shadow work beneath the left eye (left to us, Zoe's right eye) so that side of her face is looking a little flat. I haven't had time to work much on it today, but fixing that helped a little, as did shortening the nose just a wee bit.
Thanks for the tip of using multiply. I just looked it up, and it's probably something I should be using instead of the skin tones document palette I set up. I already did a little work on Emily's skin, but it's not too late to try the multiply blend on her skin instead. Thanks.
I showed them both today. Zoe hates it and refuses to look at it! But she's eighteen, and tells me she hates that photo (in fairness she's got quite a strange pose --- one she probably thought was "grown up" at the time). Emily hugged me and said "I love your picture, Uncle Tim." Not sure how to take that. She might be only ten years old, but she learned the arts of sarcasm and being patronising years ago! Well for a first effort, I don't think it's too bad so far (I'll probably look back in a year and cringe!).