The higher contrast after brief drying arises through the evaporation of water, so that the colour particles remain more concentrated. Ink-killer-compatible royal-blue inks are based, however, on a dye that is converted by the ink killer into a state invisible to the eye (ink death), which is why here limits to the achievable saturation exist. A school-suitable royal-blue ink with simultaneously high contrast is therefore chemically limited. The cause is the interplay between killability and colour intensity.