@Solander, Yeah, you're right. The black plastic doesn't work either. The light is already diffused, it's just the colour is too light. I'm looking for a dark red glow. At the moment it's scarlett, almost pink. It's not very Klingon!
@smusno, Cheers buddy. I'll give the cotton a try. I'm not trying to diffuse the light though, I'm trying to change it from a light red (scarlett) to a dark red. I don't think it'll be possible without a dark coloured material.
Of course, if I was doing this the expensive way,
I'd buy some lighting gel or colour filter gel, like they use in the theatre. I might just end up doing that since it would come in handy in the future.