Thank you for the example, @shiv0r. The links are really usefull, @dvlfsh thank you too! That´s really motivating, @stealcase 
Ok so I tried out the smudgetool and the mixerbrush. I found a way to get in the direction I wanted. During rendering I already noticed that the shadow is too low but I kept going because the effect was in that moment more important for me. So it probably doesn´t make a lot of sense but I learnt sth, so I will keep the thing I did wrong in mind for next studies. I think there is a lack of valuedifference too what makes the form less readable, well, next time
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that was the reference I was working from: 