Quick answer: GameMaker vertex shader complaining about attribute count mismatch? Default vertex format has position+color+uv; shaders must consume all attributes.
Custom shader uses position only; warning: 'in_Color not referenced'.
Consume all attributes
Reference in_Color even if unused; suppresses warning.
Or define a custom vertex format
vertex_format_begin with only position. Format matches shader expectation.
Verify with shader compile log
Log shows attribute usage. Unused = warning.
“Shader attributes must match the vertex format. Mismatch warns; sometimes errors.”
For custom shaders, define matching vertex formats. The warning is informative; mismatch is the bug.