I remember a school science experiment where you put different substances on one end of a piece of soaked blotting paper and watched as they separated out into different colours. As I recall, this was called chromatography and I think it works because substances, like felt tip pen ink, are often made of different particles with varying sizes and weights.
The ink pen I do most of my sketching with (a Parker fountain pen with Parker ink) comes out black but water reveals it is actually made of blue and orange tones:
I did this sketch on pre-wetted paper, teasing the separation out further at the bottom. My aim for this one was to try and capture the same loose style I’d admired a few days earlier at the Lui Shou-kwan exhibition at the Ashmolean.