I think you could slightly increase the contrast on the two lighter skinned guys on the sides, the tanned guy looks nice. Perhaps also give the hue of the shades a bit of a twist, but I wouldn't go overboard with it at this size, unless the piece around them asks for some dramatic colored shading.
Skin hues, eh, that is a can of worms, skin is a rather complex surface regarding how light is reflected/absorbed on it. While we wont be emulating all the light shenanigans of skin at this scale, (it is usually safe to treat it as just any opaque object, while minding the ambient diffuse light), these peculiarities still lead to many hue variations which should be explored when drawing, say, portraits, like when the beard area on pale man can have a greenish tinge, cheek blushing (never got it right) or like when people got some natural purplish shades around the eyes. If I remember well Arne had a great tutorial on skin which wasn't just for pixel art.
