I'll admit that's outside my primary area of expertise. However:
Obviously, you could actually replace the glass, rather than using "window wallpaper." It really depends on the home style, but perhaps either with colored glass or stained glass.
You could etch a design on the glass.
You could put a wreath on the front door. That's not altering the glass, but it's still altering the appearance and adding color.
Depending on what's surrounding the glass, you could address that--same as a picture frame having a major effect on the picture it's framing. So your solution might be to stain or in some other way alter the area surrounding the glass.
Along the same lines, what's surrounding the doors themselves? By altering that, you can draw more (or less) attention to the doors themselves. Or by color-coordinating the surroundings with the door frames, you can introduce more color into the entire picture.
Consider the hardware, too. The door handles. You might be surprised at the effect that new door handles will have on the entire front door area.
Another idea (I'm sure it's not original with me, but I haven't seen it suggested anywhere): You could have miniature colored spotlights from the inside of the house directed at the glass panels. Keep it subtle, and it'd probably work far better with frosted glass than with clear glass. But that'd be a great, inexpensive way to add color.
Remember that while you want the front appearance to be inviting, and to set the tone for the rest of the property, you don't want it to be jarring or feel out-of-place.
Here are a few sites I ran across online that may give you some additional ideas: