Waterman Mysterious Blue ink

For some reason I cannot explain, my Santini Italia Calypso fountain pen, full of Waterman Mysterious Blue ink in its piston-filled reservoir, will write in a very different — and, mark this, lighter not darker — colour after being capped and unused for several hours.

I decided to extract the ink from the reservoir, and compare it against ink still in the bottle from which the pen was filled, by chromatography to see if some contamination or strange chemical reaction may have altered the constitution of the ink.