Unlike house paints, artist's acrylic paint peels easily from smooth glass. Use it in thin dilution for more light penetration or straight from the tube for complete opacity. You can brush, stipple, sponge or spray it on for a variety of effects, but spraying gives the most uniform color and texture. Any acrylic paint will work, but the liquid types designed for crafting are least permanent and the easiest to remove -- something to bear in mind if you are renting.
If you like stained glass windows but have neither glass-working skills nor the pocketbook to buy stained glass, make your own with translucent liquid polymer “clay” instead. Use colored pencils to color a design from a coloring book or drawing, then paint over the finished paper with a brand of translucent liquid polymer such as Sculpey, Fimo or Deko Gel. Bake as directed, then wash off the paper backing for a flexible decal-like artwork you apply directly to the window. Friction holds it in place until you remove it. Or, pigment the polymer liquid with oil paints and bake it directly on glass panes -- a great alternative method for small, removable window panes.
One of the easiest and cheapest paint-on glazes for windows is ordinary white glue tinted with food coloring -- for complete transparency -- or acrylic paint if you want a translucent effect. Since the glue makes the paint appear milky until it is dry, and a small amount of color goes a long way in glue, mix test batches and try them on scrap glass before painting a window.
A truly novel way to create a translucent window pane for privacy is with a cornstarch and water solution. Cut thin paper or a freshly ironed new white sheet into pieces the size and shape of the windows you plan to cover. Mix up a thin batch of cornstarch in water and paint it over each sheet while holding it against the window pane. Smooth out air bubbles and wrinkles as you go. When the starch dries, you will have beautiful white window panes that let in soft diffused light and keep out prying eyes. Best of all, a bit of hot water is all you need to remove them when they are no longer wanted.