The different color caps are on the "allegedly" foil-sealed newer ones that leaked. Most all of the newer caps are on the black of the pigment ink set recently ordered. I don't know which blacks ones as they are filed away now in baggies.
The nippled ones are all the clear type as old and were on all of the colored inks (Yellow, Magentas, Cyans.).
Just the MK black was the messy one that leaked badly.
Fwiw, I get a lot of dry-eye eyedrops in those nipple dropper bottles and they are sealed with tear-off shrink tape at the cap to bottle joint. Pain to remove, but none have ever leaked - so far.