I could paint these all day!
Red paints used right out of the tube are intense and a little artificial looking. Nita Leland (Exploring Color) suggests altering the intensity of pure color by adding a bit of burnt sienna. In the book Mixing Your Own Watercolors, John Lidzey achieves this by adding a very small amount of the complimentary color green. As you can see from the eggs, adding blue/green creates a very different result from adding a yellow/green (cool/warm).
Actually, I added too much green to the mixture, which did more than modify the reds, it significantly changed the colors. But you get the idea, and any of these could be used for the shadow area.