There’s an app called ‘Sketch’ by Adobe. It features some great traditional drawing tools, like pecils, pens, brushes etc.
But there is one specific tool which really stands out when it comes to mimicking the real thing. The watercolor brush (there’s actually 2 variations of this tool) does a great job in simulation the flow of water and paint. It lets you paint wet in wet, you can then let the paint dry (see the ventilator icon below the tool box) and start stacking layers of watercolor paint onto eachother.
It took me a little practicing, but after some time I was able to achieve some satisfying results:
