Hi Fulcanelli,
This effect you discovered, is the shading we are talking about in inks. When an ink shows darker and lighter colours when writing, essentially when thinner and thicker or wetter and drier lines or parts of lines are laid down, it is called shading.
It is a loveley effect, IMO anyway, that, once you've discovered it, want to happen in all inks you use

. This is one of the reasons I use many pens in my ink reviews, in order to try and show the shading characteristics, if that ink has any.
Many of the Noodler's inks show this characteristic to great effect, and it is one of the reasons why I like Ottoman Azure and Cayenne so much. Waterman Blue Black and South Sea Blue do this too, but to a lesser degree.
Especially with flex nibs and italic nibs this effect becomes very noticeable and pronounced, and it is one of the reasons why the same ink looks different from a pen and nib combination with different physical flow characteristics.
HTH, warm regards, Wim