Specific object collision

Hi all, I have set up a collision script so that one a specific game object "missile" collides with another game object "roid" a particle is instantiated at the "roids" position. For some reason no matter what object collides with the roid the particle is instantiated. How can I get the particle to fire ONLY if the objects colliding are the "missile" and the "roid"? Here is the code I am using for this:

var roid: Rigidbody; var missile: Rigidbody; var missileExplodeFire: ParticleEmitter; var missileExplodeFireRed: ParticleEmitter; var missileExplodeRock: ParticleEmitter;

 if (collision.gameObject.missile == collision.gameObject.roid)
 {
     missileExplodeFire = Instantiate(missileExplodeFire, roid.position, transform.rotation);
     missileExplodeFireRed = Instantiate(missileExplodeFireRed, roid.position, transform.rotation);
     missileExplodeRock = Instantiate(missileExplodeRock, roid.position, transform.rotation);
     missileExplodeFire.emit = true;
     missileExplodeFireRed.emit = true;
     missileExplodeRock.emit = true;
 }
 if (!collision.gameObject.missile == !collision.gameObject.roid)
 {

 }

}

Any ideas would be greatly appreciated.