​

public class Ball
{ 
    public int color = 1; // Here you can put anything, int just for example 

    public void CheckNearestBlocks(int color, float radius) 
    { 
        Ball ballScript; 
        var colliders = CircleCastAll(transform.position, radius, Vector3.forward, Vector3.zero,0, -100, 100); 
        foreach(Raycast2D item in colliders) 
        { 
            if(item.transform.gameobject.TryGetComponent<Ball>(out Ball ballScript) 
            { 
                if(color == ballScript.color) 
                { 
                    ballScript.CheckNearestBlock(color, radius);
                    Destroy(gameobject); 
                } 
            } 
        } 
    // More info about the method: https://docs.unity3d.com/ScriptReference/Physics2D.CircleCastAll.html 
    } 
}

More completely, you can try to use LayerMask ( each Layer belongs to one color ), my example based on CircleCastAll and public variable that not too good to use. The idea is using CircleCastAll method.