I'm not sure if this is intended behavior or not. When fluid wagons are colored according to their contents, they become translucent. In the screenshot, you can see the base of the pump for the station through the front end of the fluid wagon. Once the wagon is empty, and is reverted to the default train color, it becomes solid again.
Screenshot: https://www.dropbox.com/s/o42ijfsodwz2bhu/2022-06-09.png?dl=0

Hi there, please send me a list of all mods you have enabled here so I can attempt to reproduce this issue. Thanks!

Here's an fresh save game to sync the mods to. :)
https://www.dropbox.com/s/tbzdh3x0z2den5t/NulliusTest.zip?dl=0

I was able to reproduce this for Nullius Fluid Wagons 1-3, while Paint Fluid Wagons is enabled in the settings. It appears to be due to these specific wagons color attributes, which are set in their prototype file with an alpha=1 value and simply recolor the sprite accordingly. When receiving the fluid color via this mod, the alpha value is overridden, and since all color settings within are fixed at alpha=0.5, the wagon becomes transparent. The simplest solution would be to disable fluid wagon painting, but I'll see what kind of exception I may be able to add to this mod to alleviate the issue. It's quite the edge case you've discovered!

I'd also note that for any mod that allows transferring colors from a locomotive to a fluid wagon via the right-click copy/paste method, the Nullius fluid wagons will again become transparent as all locomotives painted using the in-game interface have a default 0.5 alpha value.

I'll poke the Nullius side too. Thank you for your investigation. :)