Can you post a larger image? I'm having trouble seeing what you've got going on there...
Usually, when dealing with yellows, you do want to put some sort of purple in there for the shadows & shading. But it has to be the right purple, and it helps to layer it in early. It sounds like it's still pretty much on top without any yellow added back in?
Because your greens are so strong on the stems & leaves, some greens might work well in there, too. As well as maybe (maybe! test swatches first!) some browns?
In some test swatches, try to duplicate what you've got happening already and then see what happens if you add a brownish-orange on top of the purple, and then add some yellow on top of all that. You might be surprised (in a good way!) at what you get.
Figuring out the right color combinations can be a bunch of trial and error test-swatches.
Another thing to keep in mind is that the shadows/darker areas are there to help make the bright/highlight areas really sing. So having the base of the tulip less bright is what will make the overall bouquet seem bright and cheery. Not that you want it muddy-- Just remember that area isn't the star. The mass of yellow that makes up the body of the flowers *is*.
Hope that made sense! And I'd love to see a bigger image.