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.
Here are the (Sunday) Biscuits
This word has proved too long to keep whole lot in focus - Hummingbird - John Mitchell 736 EF nib