Alcohol markers are used by many people who just love colouring books. The downside, is that most of it is printed on paper that is thin enough to cause “bleed through.
The upside to using alcohol markers is that has excellent blending capability and colour range. Crafters use these types of pens as they give seamless colour and don’t streak.
Popular brands: Ohuhu (which are my favourite), Spectrum Noir, Promarker, Copic, Touch, Chameleon.