Backlighting
It is hard to tell, but if you look at the screen capture of the cel, there is a bright light shining behind it. There is no background for this cel.
This is a scan of the back of the cel. You can see that it is covered with thick black paint. The artist did this so that when the bright light was shone on the back of the cel, all the irregularities of the different paint colors and thickness would not show up on when filmed from the front.
You may be wondering what the white patches are. They are pieces of paper that are stuck to the black paint. After filming, this cel was placed on top of a piece of sketch paper (most commonly its matching douga), and the stickiness of the acrylic paint stuck them together. Somewhere along the line, someone tried to remove the stuck paper. Since the paint tends to adhere very strongly to paper, bits of the sketch came off with the cel. I am pretty sure this completely destroyed the douga, which is why this cel was sold without it.
The above setup is also backlit, but the situation is a bit more complex. In this case there is a background. There is still a bright light shining in back of everything though. If you look out the windows of the scanned cel, it looks like a cloudy sky. That is an illusion I created with my scanner. The window panes are actually holes that have been cut out of the sheet of paper that the background is painted on. I then layered a second background of a cloudy sky behind the first background. You can see truth when you look at the screen capture. In the show, there is only bright light visible shining through the window. That is the backlit part. Since the background blocks the bright light from reaching the back of the cel, there was no need to put black paint on the back of it. The back of this cel is multicolored, just like any other cel.
