In total there are 9 different possible section colors that you can select with the production models of the Vertex ranging from clear, smoke gray, blue, red, orange… even to a “Coke Bottle” material that is similar to the color of the vintage glass used to hold the soda. Regardless of which color you choose, you will be able to clearly see the ink sloshing around in the pen when used as an eye dropper, but the various colors will limit being able to see the ACTUAL the ink color itself. If showing off what ink is inside is important to you, I’d recommend either the clear versions or even the smoke grey. In the black delrin barrel version I have the smoke grey section and the rich wine color of the Iroshizuku Yama-Budo (mentioned before) stands out clearly.
When it comes to sealing up the pens, the guys seem to have it covered with at least three o-rings. (I’m pretty sure that is it…) One in the cap that connects the section and cap while closed to help reduce the nib drying out, one in the section where the nib threads in to help keep a tight seal there, and one more inside the barrel where the section goes in so that when eye dropped the pieces stay together and don’t leak. I appreciate them going to this level of trouble to make all those things happen. When using other pens as an eye dropper, generally you have to add silicone grease to all the parts to be “safe”, but this makes it a bit more plug and play.
I did notice on a couple of occasions that ink was in the cap of the pen when pulling open (it is a fountain pen) and then got back on the section when closing, but not sure if the tight seal of the cap o-ring produces some suction. Fountain pens tend to make inky fingers to some extent, so I don’t necessarily see this as a huge problem.