Here are some results I got:
The steps for black ink are as follows:
In Photoshop, put the bottle image onto a layer, and duplicate this layer. To do this, you can select the layer in the Layers panel and press Ctrl+J on Windows or Cmd+J on Mac. We will work with this duplicate layer only, which must be above the original.
Use your favorite method to select just the glass of the bottle. You should have your duplicate layer selected in the Layers panel at this point (if not, then select it). Once you have the selection around the glass, continue.
Click on the “Add Layer Mask” button at the bottom of the Layers panel. Since you had your duplicate layer selected and also a marquee selection on that layer, this should create an opacity mask that reveals only the glass part of the duplicate layer. You won't notice that anything has happened, but the layer mask will show a white shape (the glass). Now click on the icon of the duplicate layer so that we’re not working with its opacity mask.
Using the main menu, select
Image > Adjustments > Levels... and set the three slider values (left to right) to be 68, 0.10, and 255. Leave the output levels alone. This should darken the glass showing through in the duplicate layer to about the consistency of black ink.
Now take a size 45 brush with 0 hardness and set foreground color to black. Then select the opacity mask on the duplicate layer, and carefully mask the edges of the dark glass shape with the brush. You will want to do a little on the sides and a bit more on the bottom because there’s more solid glass there.
Using a much larger brush, like 500px with 0 hardness, or the gradient tool with foreground color still set to black, carefully mask the bottom of the dark shape and also the sides. The bottom should receive maybe a bit more than the sides. This step is crucial because a container with dark liquid will reflect its immediate background. That’s the effect we’re trying to create, as though the dark glass is reflecting the gray surface on which the bottle sits.
That’s it. Enjoy your bottle of ink. Keep in mind you can play with how much you mask the dark shape. I maybe went a bit overboard. There is a sweet-spot there somewhere, I know it. Just use the edges of the soft brush so you're not erasing but just fading slightly.
To create some other color substance inside the bottle, it's pretty much the same process, you can just play with the Levels a bit. The Levels dialog has a "Channels" dropdown option. If you select the individual Red, Green, Blue channels, you can create almost any colors and then go back to the RGB option to adjust the darkness to your linking. If you want a more solid look to the ink, you should use the Clone tool to take out anything seen through the glass, like the little bits in the middle, which I took out in the blue ink version. By the way, you can use Hue controls to change color, but I would leave Saturation alone, since it highlights compression artifacts.