This is my process of digital watercolor painting it's a quick and easy method.

I hope this will be helpful for you.

Layer Concept

Step1. Import mid-gray watercolor paper scanned to top layer

Step2. Overlay mode, 30-50% opacity

Step3. Create a normal layer for line art

Step4. Create a multiply layer above line art for coloring

Step5. Create a multiply layer again for second coloring

This is simple and high quality brushes for digital watercolor painting.