Skin is a tricky thing to paint. A great first step is research and observation, and this video presents a fun exercise to help you practice. It's common for us to think of skin as a color, but it's strongly affected by the current lighting and never ends up looking like those "skin color" crayons made for children. To make things even more difficult, skin has a way of changing its hue as it passes from light into shadow. All of this tends to trip up beginners, and it's common to have your skin tones look lifeless and cold.
I encourage you to try the exercise I mention in the video, and Cinema Squid is a great place to find high resolution screenshots. Have fun!