I wrote this a while back, for a painting site, but its still relevant:
How to paint Glass (or anything else!)Glass seems tricky - at least until you realise that you all you have to do, to paint it,
or anything else, is put down spots of the right colour in the right places - for example - here is a photo of a glass:

Let's have a closer look, zoom in a bit:

Hmm - pixels... - and a bit closer still:

Still just about holding up the illusion that it's glassy, and that we are seeing some of the background through the glass - but let's get closer still.

- now it's apparent that there's nothing "glassy" about this at all -
it's just a lot of little coloured squares 
, which "report" what colour the camera saw in that particular area.
Now those are pixels - but one can take the same approach with dabs of paint!

(or areas of light and dark shading)
Dave