Fabianv:
It looks like you’ve got the blending technique down pat, but you’re painting is lacking saturation. The animated gif I put up demonstrates the point Linda made in her notes about letting your brushes make all the blending choices for you. You’ll notice that at step 3, the gradient area has lost all sense of saturation, so it’s looking very dull. Adding that awfully hot pink to the transition area makes the formerly dull transition look a lot more vibrant. Granted, the color choice was very exaggerated, but the point stands.
In the case of your painting, the shadow areas, in particular, are very desaturated and, hence, muddy-looking. You also have a fairly low range of values. Try pushing the lights and darks more, and adding some saturation back into the picture.

