Itch Art has a neat, in depth tutorial on just about everything. Check out the entire thing here: http://www.itchstudios.com/psg/art_tut.htm

There's really just one kind of light. It bounces. You can only see the light (photon) if it enters your eye. Light does two important things when it hits a surface. First, a part of it is absorbed. This is how colors are made. A red apple reflects mostly red wavelengths, the rest are absorbed and turned into heat or something. That's why black stuff get so hot in the sun. Anyways, the reflected light bounce away differently depending on the surface. If the surface is bumpy it will bounce away sort of randomly, like a tennis ball that hits rocky terrain. If the surface is smooth it will bounce away in a predictable path. A mirror is very smooth so the light comes back undistorted, so we can see our reflection.
Note that all surfaces have speculars, because speculars is just reflected light. It's just more broken up/diluted on dull surfaces.

Depending on where the eye/beholder is, it'll see different light and different specular spots on a curved surface such as this. A puddle isn't curved (other than the edges because of surface tension) so you'll only get a shiny reflection from a certain point of view. Point speculars can only appear in an environment where there's a point light source, like a sun, lightbulb or small window.

Photo - Speculars do exist on cloth, diluted and subtle. I stretched out my shirt sleeve with two fingers to get a flat surface between the two marked dots (I moved the camera and not the sleeve).

Here on earth we have lots of stuff around us that the light can bounce off, so things here are more or less lit from all angles. For example we have the sky which is like a dome shaped blue light source. Then theres the ground, walls and other surfaces. In space there's basically just one light source, the sun. This is why the moon just has a lit and shadowed side, and looks kind of flat. If you looks carefully however, you can see earthlight on the shadow side of the moon, but it's very weak. Then there's starlight, which I guess is even weaker.
When light hits a surface and bounces, it also change color. If it hits another surface of the same color it bounced off, it will make that surface look even more saturated.


(Too orange to be some sort of skintone anyways.)

Exposure

The sunlight is much stronger than the skylight, which is in turn much stronger than indoor light. Our eyes adapt automatically after a while, and we can also adapt by squinting or just focusing on an object. Because we do this without thinking about it, it's hard to understand that our eyes are actually kind of limited. This limitation becomes even more obvious with cameras. If you take a picture indoors, the windows will become overexposed (bright). You might try to adjust the exposure levels to the window light, but then the indoor environment will become underexposed. This can be used to your advantage. By for example putting a character or object in the foreground where it's darker, you can make the silhouette read well against a well lit room.
The exposure to light can also make parts of a body look very bright or dark, not skin tone color at all. When the shadow is dark and the lit side is overexposed, the only place for the color to go is on the edge between them.

Materials

Here's an example of various materials and how i render them.

• Cloth - Hardly any speculars, just shadow and light. Sometimes strong light can penetrate thin cloth and cause some sort of subsurface scattering.
• Leather - Can be a little gloss and thus have a few speculars. Also, don't make it too saturated.
• Trees and wood - Dull. Not very saturated either (sort of grey-brown-sienna).
• Stone - A bit like cloth. The surface is often to rugged (both at micro and macro levels) to have any serious speculars.
• Plastic - It seems like the speculars and reflections are colorized in the color of the plastic. Plastic can also be a bit transparent.
• Gold - Gold isn't orange. I use black - desaturated orange sometims with hints of green, then up to yellow and white.
• Silver - More or less like a mirror.
• Metal - In the case of armours I often push the values a little more, not as much midtones.
• Brushed metal - It's sort of like an inbetween of a grey surface and a silver surface.
• Glass - Often just transparent with distortion. The speculars come suddenly and are often white. In the case of car windows you might have noticed that it's easier ot see what's behind if there's a shadow over the window. The brighter reflections obscure.
• Wet stuff - more speculars, can become transparent in the case of cloth, and stones get more saturated and pronounced details.

Shadows

Shadows are quite flat and generally less saturated than the lit side. It's easier to notice ambient light in the shadows. Shadows get blury over distance, this is called diffraction.

(Shadows don't add (multiply) with just ONE lightsource that is...)

Skin tones

Consider the environment. The light is stronger outside, and the skin color tend to be less saturated due to the sky blue ambient light and sky blue speculars. Sometimes the skin color become shifted towards purple because of the sky blue being mixed in. This is especially true if the subject is standing in a shadow.
Indoors (no windows, only light bulbs) the light is warmer and allows skin saturation to be amped up to oranges and reds.
The shadow color of the skin can sometimes wander off to greens, especially if the room have green components, like wallpaper, plants, furniture.
In a white room or a bathroom the skin tones would be quite pale, closer to local colors and less contrasted (shadow/light) due to lots of ambience.
A room with a single strong light source will probably result in near black shadows.
...so, the type of environment your character is placed in very much affects how you should render it.

Hues

The human body has a lot of different hues. Parts covered by cloth gets less tan. Mons pubis, the hip bone area and the chest is quite pale. The shoulders and lower arms gets a little extra tan. The inside of the lower arm is often pale however. The kneecaps and elbows have a little darker skin. The face also has a lot of hues, such as rosy cheeks, males might have grey or almost green jaws because of stubble. The best way to learn the hues of the human body is to make studies of course. Don't forget that animals, monsters and objects also have hues. If you paint everything with the same hue and saturation it will look boring.
Some hues are due to ambient and reflected light. The shoulders and surfaces pointing up can get a blue hue because of the sky reflecting.
Saturated gradients - The gradients between the shadow and light is not just an in-between color of the shadow color and light color. If the shadow and light is just blended, it will look very lifeless. If you look at pictures you will see that the gradients is saturated. It's especially easy to notice if you remove that saturation.

Sub-surface scattering - Strong light can penetrate the surface of some materials and bounce around, then exit again. This will increase the saturation and make the surface look illuminated from the inside. In the case with human skin, we sometimes see it on hard edges between light and shadow.

Photo - Leafs are gloss on the top side which means there can sometimes be a sky blue specular here. The light shining thru the leaf makes the bottom side more saturated, this is also true for ears and fingers, which can turn super red when heavily backlit.
Photo - Sub-surface scattering on the fingertips. The light on the left side of the thumb is probably light reflected off the index finger.
Photo - Note that the edge only appear if the light is overexposed. It does not as pronounced on the thumb.