TMcD - Thank you!
Docsaro - Thanks! On the underpainting... I first saw purple under painting done in watercolour portraiture to bring colour into the shadows. I forget the name of the artist at the moment but it was a woman here. I'll get the link for you if you like. I've done maybe around 10 portraits or so in that method in watercolour. When I switched back to working in oils I just decided to continue with the purple.
I've lived with the question "what the heck is the purpose of an under painting" for years. I found lots of techniques but no one really made the purpose clear to me.... until.... much later. Well, now I know the under painting serves as the base beneath layers of glazed paint. I did start off each of my recent paintings intending to glaze them but now working on the under painting for the third I'm telling myself... who am I kidding. I'm not going to glaze this. I might start of with a glaze but I'll go right back to opaque paint very quickly. I tell myself that purple is keeping my colours in harmony, but I'm not really sure how much really shows through. I do use purple when I mix flesh. (Another question few seem willing to answer.. how do you mix flesh.)
I do use the under painting to hopefully sort out my tones. I have difficulty deciding tones when I'm painting in colour. I probably get them wrong in the end anyway.
Hope this provides some answers for you. Again, if you want that link I'll be happy to find it.