(Susan M. Pigott is a fountain pen collector, pen and paperholic, photographer, and professor. You can find more from Susan on her blog Scribalishess.)
Lamy Dark Lilac ink is a special edition formulation designed to complement the new Lamy Dark Lilac Safari fountain pen (which I do not own). It looks spectacular in my TWSBI 580 AL purple, however.
The ink is a true purple, meaning that it is neither too red nor too blue in tone. Some purples, such as Pelikan Edelstein Amethyst and Diamine Imperial Purple, have a red tone whereas others, such as Diamine Damson shift more towards the blue/grey spectrum.
Dark Lilac is a deep, rich purple but not so dark that it appears black. It is a wet ink and flows well from my TWSBI 580 with a medium nib. It writes without feathering on both Rhodia and Tomoe paper and dries fairly quickly, though it is definitely not waterproof.
In large nibs, the ink offers nice shading.
There is also a hint of gold sheen, but it is difficult to see unless you view it in direct sunlight or bright light.
The Lamy bottle is unique in that it has a plastic bottom that holds blotting paper you can use after filling your pen. Word to the wise: put the cap back on the ink bottle before attempting to tear off some blotting paper lest you spill dark purple ink all over your counter and hands. A friend told me so.
The bottom of the bottle also sports a deep well so you can dip your pen into the deepest part of the bottle to collect as much ink as possible.
The bottle is quite large and holds 50ml of ink. It isn't the prettiest ink bottle design, but it suits the utilitarian aesthetic of Lamy.