Hi Schmitzi, I really like your "15-20min" studies from the last page, and nice pics from berlin btw.
For your last painting: Please stay away from white for the background, because it's very hard to see the right values and the skin tones depend very much on the surrounding.
Try to mix some colors from the background into the shadows of the character (to unify) and use the "light source" color for the highlights. Hope it helps! (ich ha's irgendwo mal glese, s'funktioniert aber nöd schlecht für mich)