Change material color of an object C#

using UnityEngine;
using System.Collections;

public class hfsgthd : MonoBehaviour {
	public Color color = Color.black;

	void Example() {
		color.g = 0f;
		color.r = 0f;
		color.b = 0f;
		color.a = 0f;
	}

	void Start ()
	{
		gameObject.GetComponent<Renderer>().material.color = new Color (color.r,color.g,color.b,color.a);
	}

	void Update() {
		if (Input.GetKeyDown (KeyCode.G))
			color.g = color.g + (float)0.1f;
		if (Input.GetKeyDown (KeyCode.R))
			color.r = color.a + (float)0.1f;
		if (Input.GetKeyDown (KeyCode.B))
			color.b = color.b + (float)0.1f;
		if (Input.GetKeyDown (KeyCode.A))
			color.a = color.a + (float)0.1f;
	}
}

It works but doesn’t modify the material.
Can you help me?

you are never reasigning the material.color to the new color. You are changing the color values, but not applying to the material.

  void Update(){
       if (Input.GetKeyDown (KeyCode.R)){
           color.r = color.a + (float)0.1f;
           //Assign the changed color to the material.
           gameObject.Renderer.Material.Color = color;
       }
    }

Full Script:

using UnityEngine;
using System.Collections;

public class hfsgthd : MonoBehaviour {

	public Color altColor = Color.black;
	public Renderer rend; 

	//I do not know why you need this?
	void Example() {         
		altColor.g = 0f;         
		altColor.r = 0f;        
		altColor.b = 0f;         
		altColor.a = 0f;     
	}      

	void Start ()
	{       
		//Call Example to set all color values to zero.
		Example();
		//Get the renderer of the object so we can access the color
 		rend = GetComponent<Renderer>();
		//Set the initial color (0f,0f,0f,0f)
		rend.material.color = altColor;
    	}      

	void Update() 
	{
		if (Input.GetKeyDown (KeyCode.G)){  
			//Alter the color          
			altColor.g += 0.1f;
			//Assign the changed color to the material.
    			rend.material.color = altColor;
		}
		if (Input.GetKeyDown (KeyCode.R)){  
			//Alter the color           
			altColor.r += 0.1f;
			//Assign the changed color to the material. 
        		rend.material.color = altColor;
		}
		if (Input.GetKeyDown (KeyCode.B)){  
			//Alter the color            
			altColor.b += 0.1f;
			//Assign the changed color to the material. 
       			rend.material.color = altColor;
		}
		if (Input.GetKeyDown (KeyCode.A)){ 
			//Alter the color          
			altColor.a += 0.1f;
			//Assign the changed color to the material. 
    			rend.material.color = altColor;
		}
	} 		
}