@Karine-Beaumier That's super cute! The fox looks so happy! And how big is this piece? I ask because last time I tried using the brush pen all my lines were super fat even though I was as careful as I could be!
For the commission, did they ask for earthy tones? There is only one thing that pops out to me. The shadows of the fox seem like they are just the same pigment but darken with black or Payne's Gray. Maybe next time you could use a cooler color to make the shadows. For example, if the main color is orange, try shading with a light wash of red. I would test this in a little sketch before I did the painting though.
Will Terry has a great class on color that has made my paintings more vibrant! Working in Color However it sounds like you weren't trying to go vibrant.
I like it overall! So adorable!