The most important things you will need in order to create a realistic-looking skin tone is a base color, and light and dark versions of it. Add white to your flesh tone to create a lighter shade and add black to the color to create a darker shade. The effect of adding either black or white to the base color will result in shadows and highlights on the image, thus creating a realistic-looking subject. With colored pencils, the intensity of the color can be layered by blending the colors on top of each other. Markers will be a little more difficult. Use a step down from your flesh color and then use a stark line of black for shadows. The image can be left white to indicate where the light is hitting the skin.
The best way to keep the image looking real is to add the light and dark shadows in the correct way. Determine what direction the light is coming from. Imagine you are shining a flashlight directly on an image. Where is the darkest point? Where is the light most saturated? The middle of the image will be the color itself, and the black or white will be added to show what the light is doing. You will have to build and blend the black and white into the overall color while working in colored pencil. With the marker, you will not have as much ability to blend, so the shadows should be placed specifically. For example: If you have a light source that is very close, your black shadow line will be thick as well as the white space. As the light moves farther away, the black and white line will grow thinner.
You can use one of your markers and only draw where there are shadows. Leave the rest of the image white. The viewer will be able to distinguish the image by observing the shape of the empty space. This technique would be similar to carving an image into wood, the lines would be the same. It is not necessarily required to fill in all of the space of an image in order to create realistic-looking art.
Colored pencils are almost paintlike. You can saturate the drawing with thin blocks of color and keep the image soft and loose. The more you start to build the color by layering it, the bolder and richer the image will become. Good quality colored pencils will create an image that looks like it was painted. Pressing down hard is not the way to achieve this look; just keep shading until you achieve the desired intensity.