Load a transparent texture by script

Hey I'm trying to load a load a texture from file and set it to have a transparent texture:

I've got as far as

WWW www = new WWW ("file://pathtomyfile" );
MeshRenderer meshrenderer = (MeshRenderer)billboard.GetComponent("MeshRenderer");       
meshrenderer.material.mainTexture = www.texture;
meshrenderer.material.shader = ????

also how do i set the colour of the transparent colour (i'd like it to be white)

David

edit:

i've tried

meshrenderer.material.color = new Color(1,1,1,0);
meshrenderer.material.shader = Shader.Find("Transparent/Diffuse");

without success :*(