Currently when pointing at a fluid wagon and pressing the hotkey does nothing when both mods are installed.
https://mods.factorio.com/mod/FluidWagonColorMask

Would be nice to have this working.

Edit: the fluid wagons cannot be painted by this mod initially. Instead a different mod first needs to apply a color (such as https://mods.factorio.com/mod/CustomColor) - I'm assuming it's because something like the color parameter doesn't exist until the fluid wagon has been assigned a color.

The hotkey checks that something has a color before it sets it, as of 2.0.2, to prevent a crash that would occur with some entity types. Since fluid wagons don't normally have a color, it doesn't work until one was set.

While looking into this I realized the changes I made to prevent lamps from being colored actually broke setting colors when things are placed. Fixing that removes this issue, since the fluid wagon will get a color when being placed. I also allowed the hotkey to skip the color check for locomotives, cargo wagons, and fluid wagons, since they support color regardless if one was set yet. So should all be set in 2.0.4.

Nice