I was wondering why can’t i paint over another paint, as you can see, when I try to paint mud over those leaves it doesn’t work. The other problem, as you can see, i dont have the “Grass” painting on my layers seccion. Thanks 
The Paint Layers are applied top to bottom which means lower layers override higher layers in the Paint Layers list. Since, the leaves layer is lower in the list than the mud layer, so you need to clear its painting (by Shift + Left Click and drag) to show the mud.
The grass layer is the default layer. It shows when no other layer is painted. To make the grass show, you need to clear the painting of all other layers. I suggest to start with the lowest visible layer, the leaves. Select it, then Shift + Left Click and drag to clear it. You just need to repeat this for each new layer that gets exposed till the grass layer shows up.
Let me know if something is not clear.