There are hundreds of web sites with tips of building a DYI soft box or light tent, and it looks like people are using almost anything as the diffuser material, from white plastic bags to old t-shirts.
If I take a common household item that I might want to try to use as a diffuser, it is fairly easy to see how much it scatters light, how much it reflects light, etc. However, it is a lot more tricky to estimate how it affects the colours: if there is a slight colour cast, or if the diffuser blocks certain wavelengths. As we know, what looks like white to a human eye is not necessarily perfectly white...
Therefore I would like to know: which materials are good as diffusers and which materials should I avoid? Are there any useful rules of thumb in selecting the diffuser (besides experimenting with various materials)?
In particular, if I wanted to actually invest $5 in a diffuser, what kind of material should I buy? I would prefer a paper-like material, something that resembles wax paper or tracing paper.