I can go more in depth about the comments about the background and why it's important.
Think about drawing a figure on white paper with charcoal. The marks you make are black and the rest is white.
Now imagine doing that drawing on a grey piece of paper. The marks you make are black and the rest is grey. So now there is a middle tone.
If you take a white charcoal stick to made highlights or block out a background, your figure will look much more 3D simply because of the middle tone.
Concerning your piece-
The vibrant use of colors and use of white in between those colors gives this piece a feel as if certain parts are glowing. By using only one value darker everywhere else immediately those colors and whites become the brightest thing. They will actually be perceived as brighter even though they've stayed the same. It's a local color/value relationship issue.
I'd love to se something like this done on a black (or very dark) background to really see it shine.
One other thing. The way you've cropped the sunflower looks like a style choice and it works well. But I can't say the same about the leaf below. It feels unsettling how this curving organic form just gets cut off. I'd recommend in the future to make a subtle natural boarder and don't let anything get go off the edge of the canvas.
"Not conceded, just better than jail" - Dr. Spawk, FPC