Iris Shaders Fails to Compile entities_alpha (Undefined iris_vertexColor)

What it looks like in the log

java.lang.RuntimeException: net.minecraft.class_2973: Invalid shaders/core/entities_alpha.json: Couldn't compile fragment program (<iris shaderpack sh

How to fix it

1. Update Iris Shaders (along with your matching Sodium version) to the newest release available for your Minecraft version, as later Iris updates fixed attribute passthrough for geometry shaders.
2. Update your shaderpack to its latest version, or switch to a shaderpack explicitly tested for Iris compatibility if an Iris mod update is not available for your game version.
3. If you are writing or editing the shaderpack yourself, temporarily remove the gbuffers_entities geometry shader (.gsh) or adjust its outputs so vertex color attributes are passed to the fragment stage.

Check your own log