Uni Posca Paint Markers are unique in the fact that they can draw on lots of surfaces but they remain waterbased, so will not bleed through paper. They come in a variety of nib sizes and shapes, from the 0.7mm Pin (the smallest) and 15mm Extra Broad (the widest). Bullet and brush tips make up the rest of the range, which all come in up to 33 different colours.
Each size of pen can be identified not only by the difference in barrel size, but by the lid hook which is a different size/colour, so the pin tips will have a short black hook and the fine ones have a longer clear hook. I'm calling it a hook as I don't know the proper name for it!
As far as colouring is concerned, these pens are great for giving vibrant, non-scratchy colour without shadowing or bleeding on double sided books, and give brilliant coverage due to their paint-like ink. They don't leave stroke marks like normal waterbased pens, and feel very quick to use as well.
This is a picture I coloured with the paint pens, and it took a lot less time than I would usually spend colouring something as intricate as this.
You can find the range with FREE UK DELIVERY at Cult Pens.