I counted:
-3 shades of blue
-2 shades of red
-2 shades of skin color
-1 shade of yellow
-2 shades of green
-2 shades of gray
-2 shades of purple
-black
-white
You count that manually. A program count adds up to 26. Which means you need to start unifying different values of the same color. For example, Rock's eyes are a different white than the lines in the background. As you can see, Nebula Black's edit fixes the image by doing exactly.
However, more is to be done still. Are you still adhering to 16 colors even when you switch to Forte and Blues?
Furthermore, aside from just the total, you need to look at each individual mugshot. They are to be made of 2 sprites; 2 palettes with each 4 palette values, 2 of which are by definition slotted to transparency; 6 colors for each mugshot. Of course, from a quick glance, this is something the total image might already be doing.
Still, from there, the objective is to use all those colors effectively. If you compare Rock's mugshot with the others, you can tell his looks so much better for some reason. That reason is Inti's knowledge of pixel technique, color choice and anti-aliasing, among other things. Try and figure out where more or less use of colors is required and be smart about where you place each pixel.