Fade out material using alpha channel

I tried to fade out the muzzle flash of a gun by using the alpha channel of the material. It uses the Particle/Additive shader and the mesh has two materials. I tried the following code, but nothing happened

var Farbe1 = Color(255,255,255,0);
var Farbe2 = Color(255,255,255,255);
MuzzleFlash.material.color.Lerp(Farbe1,Farbe2,FireRate);

The MuzzleFlash variable is a Renderer. What could the reason for the problem be?

Look in the particle shaders. I'm a fan of additive soft. You could also put alpha channels on your textures if that might help. You shouldn't have to do anything fancy (no script anyway).