Unlike ball point pens where we are limited to the color of the available inks decided by the manufacturers. The colors of fountain pen inks are almost limitless to suit different moods, different audience or simply just for being different.
Different shades of blue, black, green, purple, red are available. If we still can't find one that suits us, we can mix our own. The blue is too light for our taste? Mix in a few drops of black. The red is too bright? Mix in a few drops of blue. We can create almost any tint that is limited only by our imagination...
Different inks also have slightly different flow rate. One that's well suited for a fine point nib may flow too freely when used with a broad point nib and one that's good for a broad point nib may be too restrictive when used with a fine point nib. Plus everything in between...