Material Lerp Var not working

I have this gameobject pingpong between materials red and cyan. But also i have another material.Lerp on another gameObject in the same script. The variables

var lerp : float = Mathf.PingPong (Speed, duration) / duration; box1.renderer.material.Lerp (cyan_Material, red_Material, lerp)

works ;like a charm, but the other material.Lerp below is conflicting with the one up top and my question is that i dont know what to do to get it working... Thanks

var lerp : float = Mathf.PingPong (Speed1, duration1) / duration; box2.renderer.material.Lerp (green_Material, magenta_Material, lerp);