I was having trouble filling a Lamy pen cartridge with the sample of Platinum Earth Brown I have --- seems to be too thin maybe? So I switched it to a Noodler's Creaper pen and was able to fill the whole thing.
Of course, switching inks to different pens means I forget which is which, so I jotted down the current pens' ink colors.
Then I compared two water-soluble inks (Platinum Earth Brown and Noodler's Red-black) for color and 'bleed'; the subtle melting of the brown ink is much more what I'm after.
An online friend recommended two other brands of brown inks to try, J. Herbin and Diamine --- good thing that Goulet Pens sells ink samples at a low cost! I just ordered 3 more to compare.