Light is color. Painting light means depending on colors - how you handle them, how well-tuned your instincts are, and looking. Unfortunelty, understanding how to paint light means understanding how to paint in general. It is all connected.
`hibbary wrote a terrific article called Thinking in 3D that I highly recommend everyone who want to improove on their artworks to read - its content very much applies to how you have to think when it comes to painting light. Light is three dimensional, it moves and bounces and hits shapes at various angles depending on where it comes from. If you do not think about this in a three dimensional room, your will not be able to grasp the idea of light and how to put it into a picture. In a way you migth say that the key to understanding light and putting it into your paintings, is to think in 3D.


I've tried to analize the different parts of painting with different kind of light, to find a red thread that goes through all kinds of settings of light. I'm sure there are many things I've forgotten or left out though.

Transparance
Shape
Color
Relationship between subject and background
Angles

Transparance
If you are working with a solid body, you migth think transparance isn't really important. But most things have areas so thin more light can penetrate it than the rest of the body. Most often it's hair or leaves or feather on birds for example.
Using transperance is an excellent way to communicate luminescence, as it often creates a glow that we all connect with light.




Shape
Understanding the shape of something, means beeing able to interpret how light will strike it from certain angles. There are many things to consider - what kind of texture does my object have, and from what angle will my light source hit it? It's a matte rof using your instincts - you alreayd ahve the basic understanding for this, for you have been watching the world since you where born, and as an artist, you are likely more attentive to that world as well. You know what it looks like when the light hits a gnarly old trees bark, you just have to think about it.
A more shapely shape however than trees, is the human body.

Let's analyze this image. The main light source comes from the upper right corner. The closer her body is to the light, the stronger the light is when it hits her. Look at her thigh and compare it to her face. It also looks like it's a rather narrow light source, so it does not hit the calf that is closest to the viewer as brightly as the thigh. Yet another example of how light is three dimensional.
Notice how her bodyshape is described by the light.
At this point, lookign at how teh light creates shadows depedning on where it doesn't reach her, you shoudl realize that there is yet another light source in this picture. Can you find it? Look at her left arm and imagine you stood in front her rather than looking at the picture, and decide where this second light is.
There is a also a faint cold backlight. You can see its effect her other thigh and the arm infront of it.


Color
Your palette will decide the light setting for your image - or the other way around.

A soft limited palette will communicate a soft general light. As you can see in these images, some contains pretty drastic shadow contrasts, but because of their color, they do not feel that stark at all, rather it helps communicate shape

A limited palette does not mean you have to make the light soft.


Bright colors can be use to communicate a various sort of things. The wet ground, a stark light source, a shape.


Troublesome things to do might be shaded situations, or how to light the color white?
Shadowy scenes might require some thinking before you start. What colro is your topic, what colors are you using to imply shadows in the background. The shadow should have the same temperature all over, but you must remember that the shadow color will effect other colors and textures differently. But despite that, all the shaded areas are connected, and this should show in your painting.

In regards to the colors white and black - they are rarely pure black or pure white, but have a mixture of severals light nuances of all the other colors around it. It can be tempting to use solid blakc or true white if you are paintign digitally, but those colors to not exist in the real world - either you have a complete lack of light and see nothing, or there is so much light you cannot see anything that way either.
Pick colors from your surrounding and mesh them together in do a dark or light surface, and shade and highlight by creating a lattice of muted colors that echo your surrounding colors.

Relationship
The difference between your subject and background can also be a tool to describing a certain light. It is a very helpful tool when it comes to creating depth and distance in an image as well. Suggest the darkness looming close, but light in the far background, or maybe there is a light source close to the viewer and topic, whilst a shadowy storm rages in the far? This is a trick I like to use in my own images when creating a sense of drama


More...


Angles
Ligth tends to flow all over the place, but depending on the sice of the light source, and its strength, you'll get very different kind of light. Often you'll have a general light that flows all over, but you might also have a very focused light that is strong and precise. In contrast you might have a light source so dim it almost casts no shadows at all, like on a misty day.


But, to complicate things, light bounces. Some surfaces reflect light better than others. The most illustrative surface I can think of is water and snow. It is like a mirror, and often it casts a tinted light as well.




The best advice I can give you is to open your eyes. Pay attention to the world around you, look and memorize. Whenever you realize that you are noticing something, it might be just about anything, stop and really look at it. Whya re you looking at it, and take the oppurtunity to study how this thing is lit, and the relationship it has to its surroundings.