The combustion engine GUI network data, the PacketFluidUpdate and the TankManager's readData() only store the fluid ID and amount, but don't remember the NBT tags, so the rendering isn't correct.

No, just no...I am not writing code to send NBT data for fluids across the network. -.-

That's true, I will make a Pull Request in a few minutes.

@CovertJaguar I don't think you have to.
the net.minecraft.network.packet.Packet class has a static method to write stacks (and its NBTTagCompound) or just the NBTTagCompound. Couldn't you use those?

Now external fluid rendering is fixed, but the GUIs still don't use the correct color.

I will make a PR in a few minutes to fix it.

Will look at them in a few minutes.
Because atm they are just fluids using the default color, because there is no FluidStack with NBT data.

What about refinery filters?