I want to change the transparency of the image inserted into Unity Sprite.

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class appear : MonoBehaviour {

    Color color;

    // // Use this for initialization
    void Start () {
        SpriteRenderer spr = GetComponent<SpriteRenderer>();
        color.a = 0;
        spr.color = color;
    }

    // // Update is called once per frame
    void Update () {
        SpriteRenderer spr = GetComponent<SpriteRenderer>();
        color.a += 0.02f;
        spr.color = color;
    }
}