I am building a castle and decided to implement a piano room with colored glass (to mock stained glass windows). I got curious about other people's creations as I couldn't find much on it, and i think the painted glass is a great feature!
I used paint on glass here and here (zoomed in): It's a very old project (that I ended up losing), but I think it's a start. The major problem I faced while painting that was color variety, since the paint tool only comes with a few pre-set colors. Here is the original image I tried to copy (I'm not a good artist, I'm a good copycat!):
Just the default colors, yeah. I think that is mostly why people don't usually do much with it. That and the amount of blocks you need to do something decent. People don't generally make rooms that big in their houses. When I decided I was going to make a cathedral, I started by setting the foundations, then this art (thinking about the sistine chapel), and only then built things around it. Sometimes I even think of doing something similar again, but I don't have much time available, currently...
The Color Wheel mod lets you get a much wider variety of colors to paint with--though, unfortunately, you have the color the blocks first and then place them, so it's not quite as convenient as the built-in paint tool.
That is enough if you have the pixel art ready. You just have to count how many blocks of each color you need.
Unfortunately, I don't really plan out builds before I do them...So it might end up being a hassle, who knows. But anyway if you build something similar again, I def would like to see it.