I don't think they are available in current stocks, but pens write up to seven lines before needing to be dipped again. Painting is implemented with the skill, and can add colors to specific words in a drawing. Basically all of the writing is 'painted' first then a paintbrush with specific colors can then target sections of sentences, individual letters, or words. Recoloring after applying can be a lot more tricky and individual, so if you color all 'a' if a word has one like 'make' you can no longer paint the whole word of 'make.' Just partial combos of letters, or single letters.
I think the arena spawn list does not currently have paintbrushes but has paint/palette 'ARENA LOAD OPTIONS' will show what you can load, so I maay just not be reading it. The arena could be good for testing stuff tho without needing to buy in character with several types of paper that can be spawned in.
Some additional materials. Not all in stock in the city but seems to be the general ranges. I may have others to test floating around.
Pages (torn from books etc) -1500 characters, which is a little weird with them described as a small note of parchment, thinking on it now.
Poster-1500 characters
Vellum scroll-1500 characters
A piece of pale parchment, fine vellum, sheet of transclucent vellum -900 characters