This forces Minecraft to ignore the brightness cap entirely. It is actually more effective than most texture packs for 1.12.2. However, be warned: This makes the game incredibly bright. Looking at snow biomes will feel like staring at the sun.
It runs significantly smoother on older hardware than modern 1.20+ versions.
For Minecraft version 1.12.2, a Fullbright resource pack removes all shadows and lighting levels, allowing you to see clearly in caves and at night without torches. Top Resource Pack Options FullBright by Beyonderboi