How can I change a shader alpha color in script?

Question

How can I get access to the Main Color and change the alpha color ? How do I know what is the variable of the main color in the shader code ?

If it's Main Color in the Inspector then this is the parameter I should get in the script "Main Color" ?

Answer 1

If the colour is exposed with the name _Color in the shader code (most defaults are), then you can access it with material.color, using a reference to the material instance you want to change, something like this:

var renderer = GetComponent<Renderer>();
// This getter copies the material to a local instance, 
// so cache it and remember to clean it up when you're done with it.
_cachedMaterial = renderer.material;

// Read, modify the alpha, and write back to the material.
Color color = _cachedMaterial.color;
color.a = 0.5f;
_cachedMaterial.color = color;

If it has a different name in the shader code, you can use material.GetColor() or material.SetColor(), passing either the internal name of the colour variable as a string, or an integer ID retrieved via Shader.PropertyToID().

Please remember to take at least a cursory search through the available API documentation or tutorials for basic questions like this - you'll often find your own answer far faster than it takes someone else to type it up for you.