This illustration, or rather impression, is created with black ink from Liquitex and watercolour on smooth all-purpose art paper (Paint-On by Clairefontaine, 250 gsm).
It was done fairly quickly and loosely to match the style of the educational website in which it appears. I always use Rembrandt watercolour paint by Royal Talens. Rembrandt watercolour has a wide range of vibrant, very high quality, and lightfast, colours. There are of course other very good brands, but I have decided to be chauvinistic, and loyally stick with this high-quality Dutch brand.