Glass fish are one of the most popular choices of fish to be dyed, although I doubt you could do it without killing them. Commercially, they are dyed by injecting them with a dye via a needle. It is said that they do not live as long as they normally would, estimations are about 6 months. I recieved some "painted" glassfish from someone who didn't want them. They are about 5 months old and are doing fine. Their color has about worn off.
I've not heard of a white-skirt tetra, but that's not to say they don't exist. You could probably tell if the color is generic looking under good lighting.