Kuretake Clean Color Real Brush Watercolour Brush Pens (Set of 6 Colours) 
 
Water-based dye colours that can be blended together when you write. They have a single bristle brush tip so last far longer than felt brush pens do. 
 
The brush tip is very flexible, just like a paintbrush, making it easy to go from thin to thick lines in one stroke and therefore perfect for calligraphers.