I suspect it's not that special but may have been checked for any strong colour absorbtion - some plastics will absorb certain colour frequencies. Many people use plastic bottles of varying types for diffusers.
You can check both for colour absorbtion and light loss by taking a series of photos with the flash in manual and use a series of different flash durations (eg 1/64,1/32/,1/16, 1/8 both with and without the diffuser. By comparing shots with equal exposure you can work out the light loss (normally between 1 and 2 stops) and by comparing the colour histograms you should be able to see if they have been changed significantly. Obviously you need to judge the diffusion achieved visually yourself.
I have found hard plastics don't diffuse too well or just stop the light, though I have to say I never bought any supposedly designed for the job. I get decent results with white synthetic stiffening material from the haberdashers. The off-white or natural fibres will colour the light.
