Warning: This is long and I apologize but I hope it helps!
So I originally typed this up as a post for friends on my tumblr but I figured I'd bring it here and expand on it a bit for comics as well. This is just a guide for different methods of light and color that you can use to bring color to life in your pages and strips. Don't underestimate the power of color for digital media! Or even traditional for that matter.
Color is something that has always been important to me as an artist and person. It helps me express not just my mood but the stories I am trying to tell. Due to this it is the subject I’ve studied the most out of the years of trying to improve my artwork and storytelling. I am no expert or teacher within this field but I figured I’d share the information and methods that I’ve picked up over time. I’ll mostly be talking about the relationship between color and light and later on the use of them within a composition.
I highly recommend looking at other studies and reading a few books on the subject as well. My word only has so much credit to it after all!
I’m going to start with the idea of Local Colors. Local colors are hues that are associated with an object or subject as they appear under direct white light. They are the stereotypes of the color world. Red apples, green leaves, blue sky. Those are all objects with their local color association.
So those are the colors I use for those objects, right? Nope! The use of local colors can actually be really tricky and make the image flat and lifeless. Your best bet is to study what makes that apple look red or that grass look green. What other colors is in the blade of grass and how they balance each other.

Notice how the image on the right has areas of blue and yellow added in to the highlights and shadows. Local colors aren't BAD but can muddle down an art work by sheer accident.
The color gray and other neutrals are those friends you love to hate. They can either enhance a piece or make it dull and bland. In all honesty, how much gray you use depends on the composition and element of that art piece. It can be great for accents but terrible for shading if used too heavily. It is just something to be experimented with and explore.
The same can go for gradients and tints. I personally find gradients perfect for underpaintings and adding a subtle change of color in a composition. But too much gradients and the piece can become confusing to the eye. Tinting is adding white to a color to raise to or give it a more pastel effect.
Below is an example of an image where I used gradients to add a subtle shading.
Ok ok enough with the boring stuff.
A good way to instantly convey a mood in a piece is through the use of a scheme or palette or colors that make up some form of relationship within each other. Often times a convincing scheme will have a mix of warm and cool tones. These tones help compliment each other and create a spark of interest within the piece.

Above is the same character portrayed in three separate color schemes. Each scheme can tell a different side to the character in how I want him portrayed.
If you aren’t ready to jump in to a full color scheme yet, color accents can help give you a start. Color accents are when there is a small dash of color on gray dominated artwork. This also works well if you are trying to make an object the focal point of the image.

Now, on to the fun part: Lighting. Light can greatly affect how a form or object can appear to us. You can easily go from a well lit innocent look to a barely lit and menacing appearance just by how you focus your light source.

Based on the lighting of the two separate images below we can guess not just the mood of the pieces but possibly gather who may be the antagonist of the bunch. It is a great way of forshadowing events and ideas.

Personally the best way of learning various light sources is to study life and photographs and to constantly push and practice. Look at what is around you and let it influence your work.
Below I will give a few examples of different lighting techniques and how to incorporate them into a composition.

Don’t underestimate the power of a cast shadow. Very easily they can be used to add a dramatic or eerie effect within a composition. Or even hint at a monster that the subject [or viewer for some cases] cannot see. Imagine a group of dwarves inspecting a treasure chest and not seeing the cast shadow of a dragon looming over them. Use cast shadows to your advantage and just have fun with them!

[image above belongs to Dreamworks's the Croods]
A fantastic form of dynamic lighting is the half-shadow. A half shadow is when you will light the top half of a composition while the bottom half is engulfed in shadow. Just be careful not to lose all your values in the bottom half and half of the image appear flat!

Edge lighting or a rim light is when a strong source of light comes from behind the subjects and creates an illumination around the form. Be wary and keep in mind that the rim is not just a sliver of white along the form but varies depending on area facing the light source.
Contre Jour is similar to Edge lighting but not exactly. Contre Jour is caused when the subject completely blocks the light source and is illuminated from behind. This allows the silhouette of the subject to be the most prominent and for the shadows to push along the subject.

Reflected light is a fun technique. Reflected light is when light is cast on a subject within a shadow and reflects its own colors.

Atmospheric light is when a subtle light is shown through composition to help reflect against the subjects and objects. It is very handy depending on the composition and mood you are creating.
Something that was a huge problem in my early work and still pops up from time to time is that a composition will feel flat and lifeless. An easy fix is to create a focal point within the composition by the use of color or lighting.

The focal point of this composition is the massive serpent’s face. This was done by making the light source emit from its face and drowning out other areas of the composition.
Another problem area would be over saturation or a muddy look to my colors that just made me want to throw it out and try again. This often happens when too many colors on a palette or too close in range and don’t compliment each other. An easy fix is to throw your palette through a grayscale feature. Do the grays all appear at different values or do they all look the same? After checking, just alter what is needed.

Honestly, a great way of studying color is to kick back and watch an animated film [or live action but animation is usually much more deliberate, vibrant and easier to analyze]. Take screenshots of various scenes and analyze what colors they used and ask why they used them. Does it have a blue color scheme? Is it a sad scene or taking place at night? You can learn a lot just by watching a few of your favorite films.
Sorry again for the length and hope it helps at least a few people out!