Change the Transperency of a Prefab in Unity

For whoever is interested, I have already solved it. Here the solution:

public GameObject cubo;

public void Transparente() //assignado a un button
{
    cubo.GetComponent<Renderer>().material.color = new Color(1, 1, 1, 0.3f); //llamo al canal alpha en el último valor (1=100%, 0.5f = 50%, 0 = 0%)
}

// Use this for initialization
void Start () {

    Material matTrans = new Material(Shader.Find("Transparent/Diffuse"));
    GetComponent<Renderer>().material = matTrans; //Crear nuevo material Transparente
}

I hope it helps if someone needs it!

The Color structure you're accessing in

cubo1.GetComponent<Renderer>().material.color.a = 0.5f;

is a copy of the one in the material. So accessing only the property "a" would involve changing the alpha of the copy, and this gives you an error (something similar happens when you try to do transform.position.x = 5)

The correct way to modify the value in C # is as you have done in your answer, with the problem that you can overwrite or lose the other color values, or doing something like this:

Color c = cubo1.GetComponent<Renderer>().material.color;
c.a = 0.5F;
cubo1.GetComponent<Renderer>().material.color = c;

In this way you leave the previous values r, g, b intact, not as in your example.

Besides that, so that you can see the effects of the alpha change, the material must be of the "Transparent" or "Cutout" type, as you have already been told.

I hope it works for you ^^

You can also modify the material, Above you see it says transparent .

Look at this example: