Anyway, hazelnuts turned out to be great practice.
I was also inspired by two books. The first one was Exotic Botanical Illustration by Rosie Martin and Meriel Thurstan. In the book the authors shows an idea for making color charts which I really like. They suggest to paint two rows of squares. The first raw is a range of two colors mixed together and the second row is the same range, but more diluted.
The second book was The Modern Flower Painter by Anna Mason. On page 7 I saw a beautiful painting of beige magnolia. I was wondering how she achieved this wonderful color and I found the answer on another page: Using burnt sienna as the base for beige will give a red-tinged beige. On occasion you might want a less red beige. In these cases, try mixing yellow ochre and burnt sienna for another great neutral mix. Simple as that! This is another reason why it's good to get to know our paints better. The easiest solution is sometimes so close!