Here you can see two pages from my sketchbook. I test the colors on the left side, and on the right side I have those which I used in the painting.
Here is a close up and a little tip. I try to mix my own blacks (not because there is a rule "don't use black paint in watercolor", but just because I very, very rarely need black color, so there no need for me to buy this color as I always manage to mix it by myself). There are many ways to mix blacks. You can mix your three primaries. Two bottom rows on the left show blacks mixed with cool primaries and warm primaries. You can also exchange your colors, use two cool primaries and one warm or any other combination. Just play with colors. Three primaries mixed together gives you also obviously a rage of grays (diluted blacks), which are called botanical grays.
Another way to make black is to mix blue with brown. Ultramarine blue mixed with Burnt Sienna gives you great black, but even better gray. I chose to mix Indanthrene Blue with Burnt Umber. This mix gave me quite a strong black which I used to paint the ends of the stemens.
I painted the upper green part. I haven't finished yet, because I have to add some white hairs using white gouache or watercolor, but I don't have it. My old gouache is too old to use it and I have to buy a new one. In this photo you can see my slow progress. Here I also have a tip. Sometimes yellow underpainting can give additional vibrancy to the leaves. Here I used Hansa Yellow Medium as a first wash. Then, when it was completely dry, I was adding next layers of my greens. Thanks to that yellow underpainting greens are more vibrant.