I don't think there's a "Color" for that. For shadows think of darkening the color by adding the complementary color then using some white to "Grey Out" the color, especially with the flower being in the background.
Get a piece of scrap watercolor paper and mix some likely colors, put a sample on the scrap paper then hold it up against the photo to see if it's close. The problem with watercolors (and the delight) is how different the colors are when you get them off the pallet and onto the paper.
You can add a light wash over the original color to make changes so long as the original color is completely dry. Use a hairdryer is necessary.
I personally would mix a different set of colors for each flower. Mix the saturated color (Highly pigmented) the dark and light for the lit side and the same with the shadow side. Then you have a start.
Remember that the viewer of the painting won't be comparing it to the photo, plus you are creating a piece of art not a reproduction of the photo, so the colors can be very different yet just as pleasing to the eye.
Look at other pieces of similar art and you might be surprised at the colors being used.
Paint some small color studies (Notams) before you start the "Big" painting, it will help get the composition, tonal range and colors right.
Hope this helps.
Brian
