I did it today - I bought a starter set of colorex watercolour inks and painted this abstract painting with it.
Colorex paints come in little bottles, together with three eye-droppers, These are used to get the paint out of the bottle and onto your palette. Depending on how intense you want the colours to be you add a bit of water. You can also mix the colours by taking a wee bit of ink with your brush and put it on some other paint in the palette or by directly dropping one paint on the other.
In this case here I used magenta and cyan, different amounts of water, and mixed them one time taking more magenta and the other time taking more cyan.
It is really an adventure, but a very enjoyable one. I love experiments!