you mean the skin? it's a combination of tweaking the subsurface scattering/translucency settings/maps and having decent lighting (and enough of it). often you see characters looking weirdly grey which is often from too little SSS/translucency or too low lighting. sometimes even if the skin settings are fine it looks wrong until you crank up the lights.
radiant does a decent job at that which gives the healthy saturation and 'waxiness' to the skin. (also I think the renders are adjusted in photoshop for maximizing the color/contast space). but you can see from the even flat shine that the specular maps are either very low resolution or not there at all. it's not a huge problem but it gives the skin that doll-like flat shine, where as more realistic skin would show slight uneven skin surface (a combination of specular maps/bumbmaps/normal maps.)
the next question is of course "how do I do that?", and unfortunately there's no easy answer for that because every character maps tend to require different settings and even methods. some skin maps work well with very low settings, some with high and so on. so you just have to fumble around with the settings, and slowly you beging to feel your way towards the right look. practice, practice, practice.
(another hugely related thing is that there's no such thing as a 'correct skin color', but instead what looks 'right' is 90% dictated by the ENVIRONMENT. I'm not talking about psychological subjective illusions, but the physical reality of a surface vs the lights illuminating it. in a certain environment skin can be even strong green, and if you render/paint it in any other color it will look super wrong. that's why often certain skin settings can look vibrant in one scene and extremely grey in another scene. you just have to train your eye to see it through trial & error, and eventually it'll become fairly instinctive. but when you get there your images will begin to look harmonius and balanced, regardless of used medium.)