I took this back to the easel and darkened the values in the upper left corner. The paint is unfriendly today, oily and slippery and everything I mix disappears into murk. I still need the book.
The light in this one escaped me. The foreground was a lighter luminous purple and the background was a deeper bluer purple. I don't know how I could have not got it, because it was very obvious and yet this entire thing is the same purple grey. All the purple turned into one value of its own accord. Who can recommend a book to read about shadows, I know there must be one out there?