FOSS gamedev and creative worlds

I have made a cauldron node, and I want make it look as if there is water inside the cauldron when you right-click it with a bucket of water. I have two different nodes made to switch between when the node is right-clicked - filled and unfilled. The problem is, I don't know how to make the middle nodebox have a different texture than the rest of the node. Here is my cauldrons left to right - unfilled and filled respectively.

The weird textures are to help you see where the nodes are.

Simply give the "full" box a different texture for the Y+ side (the first texture in the list). Make the middle of the image look like water, and the edges look like whatever the top edges of the box should be. This is most easily done by taking the Y+ texture from the "empty" one and painting the middle with a water-like image, and using the result for the "full" one.

Nodeboxes behave as though the computer assembles them into a model and then projects the textures onto the complete unit from all six sides.

Textures are repeated throughout the width/height/depth of the object, as if the nodeboxes are made of glass, so every Y+ side will have pixels from the Y+ texture, for example (ditto for the other five sides/textures), so if you have a particular colored splotch that lands in a particular spot on a whole cube, that colored splotch will land in the same spot on all nodeboxes that sit behind/under it.

Ahh! Thank you so much! It worked! Have you seen my Thaumtest mod yet? This is the mod that I need the cauldron for! :3
EDIT: Wait, one more question. Can I add transparency to the water portion to appear as if you can see through the water into the cauldron?

I have not seen that mod, no.

To get the water to look partly see-through, you'll have to "fake" it - in your graphic editor program, draw the water onto a second layer above the black/white stripes (or whatever you end up going with for the interior of the cauldron). Then make that layer, say, 75% opaque.

Export the result to a plain, flat PNG image and apply it to the Y+ side of your object, and it should give a reasonable approximation of the effect you want.

Okay, thanks. Btw, the mod is in my signature if you want to have a look at it.