Check out the time-lapse video of how I painted this here:
I used several layers of acrylic paint and allowed each layer to dry overnight. The colors white and black tend to break up a layered effect by behaving as more opaque than most acrylic pigments. Acrylic paints with a white hue also tend to be less transparent than pure colors. For example, I used a light blue wash in one of the layers, and it partially blocked the transparency, because the light blue color had some white in it already. Black was used to enhance the contrast on the edges of the flowers, so they could be made out more clearly through the layers of paint.
The process was pretty easy. I laid down a layer of color, then added white flowers bordered in black. I let that dry overnight. The next day, I covered the entire painting in another color or more than one color and followed up with more white flowers bordered in black. I let that layer dry overnight. I kept doing this until I ended up with several transparent layers of flowers.