

Just a bottle of ink some would say. Not just a bottle, but ink, the fluid that makes a fountain pen a fountain pen and helps to define the writing experience..
There are numerous brands of fountain pen inks. Each brand, and indeed the specific colours within a brand, having its own characteristics: how fast it flows, how long it takes to dry, the intensity of the colour, and yes, the ink bottle itself.
Over the years I have written various columns to express my comments on ink:
One of the pleasures of writing with a fountain pen is the ability to select the appropriate ink for the pen, and the writing occasion. Reviews of individual ink brands can be helpful is getting an idea of where to start, but in the end, the performance of the ink depends on a combination of effects.
Fountain pen inks are water-based and as such are not waterproof (except as noted in the following paragraph). Good for the pen, as if a non-water-based ink was used, the inner workings of the fountain pen would be out of commission in a short time. A mixture of water, dye and detergents, the expensive fluid is what writing is all about.
Some inks, such as those developed by Noodler are considered permanent, or waterproof, as they have a chemical in the ink that bonds with the cellulose fibres in paper. In your pen the ink is just an ink, but on contact with cellulose there is a chemical reaction and the ink is permanent.
The link that follows provides access to the reviews of numerous inks:
A brand of ink may be nothing more than the label on the bottle as today very few companies actually make their own ink. If they do make their own ink, for many of them it is a matter of mixing dyes into a base to arrive at the selection of colours.
Pens are filled with ink using cartridges, converters or piston mechanisms where the ink is held in a chamber inside the pen. Many feel that converters provide a better writing experience than cartridges (they do at least in terms of selection of ink and colour). Converters, however, if not in place correctly will cause air gaps and make a great pen perform poorly.
Writing inks are basically water with added chemicals. Dyes are used to create the various colours. Some brands such as J. Herbin from France use vegetable dyes, but must companies use chemicals.
In addition to the dye (for colour) there are chemicals that can be thought of as a thickening factor. To set how the ink will flow. Detergent or surfactant us typically added to the ink to reduce surface tension and make the ink flow, otherwise it would just blob as you drew the nib along the paper., There are also some chemicals used to help prevent things from growing in the ink.
Enjoy the option of selecting various colours is part of the fountain pen experience.