Gel Pens
Gel ink is water-based and flows freely, which is why the line reads darker and more evenly than ballpoint. The trade-off is consumption: the same page of writing uses noticeably more ink, so a gel pen runs out sooner and a floor standardised on them needs refills budgeted rather than assumed. Colour density is the payoff, particularly on coloured or darker stock. See erasable gel pens, the pen range and refills.