Actually squeezing it from a highlighter is messy and unnecessary. You can buy bottled highlighter ink for fountain pens pretty cheaply. (there are special su

Actually squeezing it from a highlighter is messy and unnecessary. You can buy bottled highlighter ink for fountain pens pretty cheaply. (there are special super-broad pen nibs for highlighting work. To use bulk ink, you have a "converter" cartridge with a piston or squeeze-bottle that allows you to suck ink out of the bottle and into the pen) Since you specifically seem to be after fluorescent, here's somewhere to get started: https://www.gouletpens.com/collections/bottled-ink?filters%5Bcustom_fields.fluorescent%5D%5B0%5D=Yes https://www.gouletpens.com/collections/bottled-ink?filters%5... Here's an "invisible ink": https://www.gouletpens.com/collections/bottled-ink/products/noodlers-blue-ghost-4-5oz-bottled-ink?variant=11884744540203 https://www.gouletpens.com/collections/bottled-ink/products/... Or a yellow highlighter ink: https://www.gouletpens.com/collections/bottled-ink/products/noodlers-firefly-4-5oz-bottled-ink?variant=11884744278059 https://www.gouletpens.com/collections/bottled-ink/products/... Pink: https://www.gouletpens.com/collections/bottled-ink/products/noodlers-hellfire-3oz-bottled-ink?variant=11884742180907 https://www.gouletpens.com/collections/bottled-ink/products/... I am not 100% sure that the viscosity will exactly match what printer ink is, but if it's too thick you could cut it with isopropyl alcohol or similar. edit: on further reflection if your goal is something that is visible outside of a blacklight, the darker colors might have more pigment and thus be more visible, although still probably quite light.