I like the idea, and the composition of the pictures, unfortunately, the tulips are too dark against the light sky. Have you tried taking this one with a flash? Also, something distracting in the background to the left (behind the still closed tulip). I know you couldn't remove it for this challenge, but I would do that for final processing.
Ah! This photo could have been really nice. Your framing is great, as is the angle. The lack of up-closeness with the flowers, the lighting of the ground, the coloring of the picture (seems washed out), and so forth are all detracting from the photo, though. I'd go back and try this photo again on a day where there are some clouds out or something to help you balance the color of the sky and also add some texture to it. I'd also try to get the flowers more in focus, and see if you can't lighten the photo up a bit in order to get the lower-green parts to have some texture. I might even lower the angle a degree or two if you can capture some dirt from the ground. I think that could add to it. Certainly worth trying out :)
A little bit underexposed, I think, the leaves and the flowers are very dark. You might have overexposed the sky if you had a longer exposure though, but maybe from a different side of the plant or a different time of day they would have been less backlighted, and then easier to expose?
I think the dimness of the lighting and colours detract from this photo and make it flat. The flowers are almost seen in silhouette, but tulips in profile aren't all that interesting, especially set off against such a dim sky.