Regular glass has a green tint to it – easiest to see when you look at the edge. This is because of the iron oxides in the glass, you can get low iron glass – its not very environmentally friendly – they use arsenic in the production process to get rid of the iron oxides. One of the commerical names for the high spec version of this glass is called borafloat, waterwhite is another one. A dag behind white glass does look a lot nicer and there are conservation advantages too – the high spec glasses are less prone to weeping – when the silicates leach out of the glass over long periods of time. Another advantage for me is that when painting a white passe partout I don’t have to color balance the white to pink so that when reverse painted onto the coverglass it appears as white should do, i.e. no green tint.