There is no simple answer, no one color that you use that magically creates 'transparent glass'. You need to forget about the objects you're painting and see the shapes, colors; to over-ride your brain's interpretation of 'glasses'. You have to observe very closely the colors that are in the objects, which I presume you have set up and are not painting from imagination.
You can't separate out the background that's seen through a glass and the actual glass itself as the background gives color to the glass. The highlights on the glasses are most likely not pure white either, as they're influenced by what's around them. Take a small piece of white card and place it next to each 'problematic' bit, to help you decide. Also look at the tone, not just the color.
Remember that when you look through a wine glass, what you see through it is distorted: stretched and squashed. It's almost as if you see the glass is there by inference rather than by a physicality.
For more on the subject, read the article on painting glass.