Wait. Wait wait wait. WAIT. There's ACTUALLY someone named "Kevin" in this comic?!
(Insert obligatory reminder that these two don't know Tedd can be a girl's name here)
The effect in the last panel was a bit of a happy accident. I originally imagined the panel entirely in black and white with some strong shadows (similar to last panel Tedd here), but I didn't like how it looked with the background, which I wanted to keep.
So I was just going to color them in all normal, but I'd accidentally included white fills in the semi-transparent shadow layer above the colors. The end result was this weird desaturated look I hadn't intended, but decided I liked. It gives the contrast I wanted while still being close to what I originally imagined.