Transparency counts as a single color, but I'm not aware of any portraits that have any.
Since some of the colors can be very close to each other, manual counting will not only prove difficult but also unreliable.
I didn't check all of them, but Pained has one too many colors:
To get here in GIMP, use Colors -> Info -> Colorcube Analysis
Grin works because it only has 13. The maximum is 15.