Here are three examples of painting glass, both clear glass and colored glass. It's actually quite fun to paint glass, and, unless you are aiming at photo realism, it's not that difficult.
There are a few principles to follow.
1. paint the background color in first.
2. very lightly sketch in the shape of the glass. For clear glass, using a color appropriate to the color of the glass.The amber glass in the still life above obviously has alot of red and orange. Clear glass might call for a more bluish tone. The dark green bottle had alot of black and green in various mixtures.
3. Light travels THROUGH glass, so the lighter side is generally the side AWAY from the light source, except for highlights.
4. Highlights occur in places where the glass either bends (as in the junction of a bottle top and the body of the bottle) or on edges (such as the top lip or edge).
5. Refractions - simply pay attention to what happens to objects that appear behind or through the glass, such as the cloth in the peaches still life. If you observe carefully, and paint what you see, you can make these refractions believable.
That's it! Glass is a fun challenge, and can make for some beautiful paintings!