Paint is an alchemy result with two green runes. Any ingredients whose runes add up to exactly that make it at the Laboratory I, and the simplest pair is 2 Water. One Flour or one Onion also works on its own, and so do two Sticks, two Leaves or two Berries. As with all alchemy, each ingredient has to be studied first.
Linda asks for one to freshen up her Dockside Club in Safe Harbor. After that it goes into Church Shrine III and into Printed Money, 10 Clean Paper and one Paint at the Printing Press. Each flask you mix also gives 2 green tech points and counts toward the Alchemist inspiration.