For better compatibility with MekaTweaker (gas) and JEI I changed the textures for my custom fluid to a manually colored in version so that JEI would display the gas variant with color. This works well; however, I had to disable colorize so that it would not make the texture even darker by coloring it twice. The issue I discovered is that if colorize is set to false then the liquid appears as a dense white mist while you are inside of it.

For reference what it looks like from inside the fluid: https://gyazo.com/4348d0302dec6f77b4a7e631fac8e566
From above: https://gyazo.com/5628ae73a6f5f8f39d0d84038c12be5a

I assume that this is a bug, however if it is not I would like to request that some way to color the "inside" fluid color gets added. As if someone wants to create a truely custom fluid texture and have it be colored inside they will not be able to without messing up the texture they have designed.

Script:

#loader contenttweaker

import mods.contenttweaker.VanillaFactory;
import mods.contenttweaker.Fluid;

var liquidDarkMatter = VanillaFactory.createFluid("liquiddarkmatter", 0x37173E);
liquidDarkMatter.material = <blockmaterial:lava>;
liquidDarkMatter.density = 8000;
liquidDarkMatter.rarity = "RARE";
liquidDarkMatter.colorize = false;
liquidDarkMatter.setStillLocation("contenttweaker:fluids/dark_matter");
liquidDarkMatter.setFlowingLocation("contenttweaker:fluids/dark_matter_flow");
liquidDarkMatter.register();

Content Tweaker version: 1.12.2-4.7.0