So the window will be looking into a dark area? with no light shining from the back? In that case, you won't be able to use the regular transparent paints "glass paints"... they just won't show up like you might want without light shining behind them. You could use glass paints that are translucent rather than transparent though--perhaps Pebeo has some like that which come in more opaque versions.
You could try though just put a sheet of white posterboard or anything white right behind the window, then use light or very bright colors of the glass paint and count only on the light shining onto the front of the paint/window. Or you could rig something up so that there was a light on in the closet, diffused and pointed toward the back of the window, but that would be fiddly, etc.
For more of a translucent or opaque version, you could just use regular acrylic paints (plain or pearly, craft versions or "artists" versions in tubes which should cover better in fewer coats).
Or you could even use (non-permanent) poster paints (with a bit of dish soap?) then put a clear gloss sealer over those to make them glossy.
You could also use shapes of self-adhesive films in colors instead of paints, I guess.
This person uses "white glue" mixed with acrylic paint for her lead lines, and "clear glue" mixed with acrylic paints for her panes:
http://www.suzyssitcom.com/2011/01/feature-friday-faux-stained-glass.html
(although "white glue" normally dries clear, but the more acrylic paint added to it--which is opaque-- the more opaque it will become)
more ways to make "faux stained glass":
http://www.google.com/search?q=faux+stained+glass
http://www.google.com/images?q=faux+stained+glass
http://www.google.com/images?q=faux+stained+glass+acrylic+paint
You could also create real stained glass or faux stained glass with polymer clay** to hang or mount in front of the window or behind it, rather than painting directly on top of the glass.
** (see links A & B below in Sources... I've exceeded the # I can put here)
...plus info, various techniques, tutorials, and links on this page at my site:
http://glassattic.com/polymer/LiquidSculpey.htm (click on "Stained Glass" and perhaps on "Cloisonne")
...and my page on Covering has a section on covering glass which could be done stained glass style:
http://glassattic.com/polymer/covering.htm (click on "Glass & Ceramic")
http://www.google.com/images?q=polymer+clay+votives
(those can be done translucent or opaque, etc)
...and to make them as solid clay with grout like "mosaics" but in stained glass stye:
http://glassattic.com/polymer/mosaics_inlay.htm (first half of page)
...and finally, you can do "paintings" directly on glass with solid or softened polymer clay, or colored liquid polymer clay, itself, which can be flat to dimensional and have all kinds of looks:
http://www.google.com/images?q=polymer+clay+painting
http://glassattic.com/polymer/paints.htm (click on "Polymer Paintings"... info is good but most links are now dead... may want to check out especially the sub-category "drawn outline under sheet of glass or acrylic, or translucent papers")
...and more sites online:
(see links C & D below in Sources... I've exceeded the # I can put here)
Good luck!
Diane B.