Since you have switched the images into the subdir liquids there is an error with the image "LiquidOre.png" which isn't found.

This results that some liquid tankes are nice black-pink colored because the texture is missing.

But I do this @aidancbrady .

I have fresh forked your repro and seted it up correctly on my PC.
Then created a new branch called "opaque" from the current development branch wich is for 1.7.10.

After adding all the stuff correctly and run gradle build all went fine. Then I putted this new JAR in my mods folder an then getting the message, that the image is not found.

Which place in the code is it defined? Because I also checked all double befor I have implemented my stuff.

Here are a pic from NEI which has some missing images:

https://imgur.com/a/WJ1ms

Items:

Clean Ignatius Slurry
Clean Cobalt Slurry
Clean Platinum Slurry
Clean Manganese Slurry
...

Not experiencing this. Make sure your assets are totally up to date, I remember I fixed this a while back.