I really like to write with Ecoline (by Talens) which as I have been told is factory premade liquid watercolor. They come in variety of great colors. They can be mixed together - either on paper or directly in the bottle, and apparently won’t curdle. For writing as such I do not recommend those light yellow ones, as sometimes they are too light to be visible on paper. They are great for mixing colors - like you just write a letter with them and seconds later apply a dot of darker color say at the bottom of it. Allow for spreading the color for itself within the damp area to create nice effect.