I have a mixture of inks and watercolours and it takes time to discover the features and differences between the two of them. I’ve summarised my findings below but let me know if you’ve also discovered anything when you’ve experimented with either - or both together!
You can get some inks that are waterproof once dry and with these, you can layer the colours while keeping crisp lines. With watercolours, the paint will become soluble once wet again so the colours will mix on the page - it all depends on the effect you’re looking for.
While Ink is almost always a liquid which is great for dipping a brush or a nib in to start right away - it’s more difficult to build up colour without layering them. With watercolours you can choose to use a lightly diluted amount vs a very diluted amount easily - it all depends on what suits your practice. Using a pen nib is also great for getting very fine details and a different technique vs watercolours where you would need to use a brush.
When talking with other artists most seem to use watercolour for washes which plays to it’s the strength of being buildable and soluble. Ink is most commonly used for linework and smaller details on top of a finished wash.
If you’re coming back to the same piece night after night - with Ink you can always guarantee that it will be the same consistency and colour. With watercolours this more depends on how it was mixed, how much water was on the brush - so there is more risk this won’t entirely match and if continuity is important - say with a children’s book/animation or concept piece then ink may be a better choice. There are also a lot of grey areas - concentrated watercolours, fluid acrylics etc.
Ink is usually made from dyes which is a pigment that has been dissolved in a solution. However, this also means they will diminish quickly if exposed to light - best to use a special UV varnish or glass if the finished piece is meant to be displayed.
Watercolour paint is pigment mixed with a binding agent - not dissolved, in this case, gum arabic so the agent binds the pigment to the page. The reason a lot of surfaces such as canvas and watercolour paper are rough is to make sure the binding is secure when the paint is applied. Watercolour paints will have more staying power - depending on the light fastness rating.
Inks can be applied to much thinner paper (with a lower gsm) as they’ll be absorbed into the fabric and dye the paper. However, if you do apply ink liberally like in washes you’ll still need to use thicker paper if you don’t want it to buckle. Whereas watercolours use water as the transport for the pigment and binder and generally you need to use 300gsm or above to make sure your watercolour paper doesn’t warp.
Feeling Inspired? Don’t take any of the rules above as gospel - give it a go! Hopefully, this has offered some advice on what to choose when you sit down to your next project.