for (int i = 0; i < trans.transform.GetComponent<Renderer>().materials.Length; i++)
{
trans.transform.GetComponent<Renderer>().materials[i] = PenMgr.mInstance.m_alphaMat;
}
Note that like all arrays returned by Unity, this returns a copy of materials array. If you want to change some materials in it, get the value, change an entry and set materials back.
Material[] newBufMat = new Material[trans.transform.GetComponent<Renderer>().materials.Length];
for (int i = 0; i < trans.transform.GetComponent<Renderer>().materials.Length; i++)
{
newBufMat[i] = PenMgr.mInstance.m_alphaMat;
}
trans.transform.GetComponent<Renderer>().materials = newBufMat;