Assigning to COLOR in fragment() gives error "Constants cannot be modified."

[saluk]

Your Godot version: 4.7 RC2

Issue description: The intro documentation page on shaders has example code that would produce the error "Constants cannot be modified.". COLOR is only an input variable to fragments and cannot be output. The code is incomplete anyway but can be misleading.

URL to the documentation page: https://docs.godotengine.org/en/stable/tutorials/shaders/introduction_to_shaders.html

[clayjohn]

That code is valid canvas_item shader code.

[Calinou]

This is a valid canvas item shader, but not a valid spatial shader:

canvas_item	spatial

In a spatial shader, COLOR is read-only in fragment (it reads the vertex color). Assign ALBEDO instead, which is a vec3 (transparency is in ALPHA).

I've opened a PR to make this clearer by mentioning the shader type in the first example:

• Mention shader type in Introduction to shaders #12063