Welcome, here I'm going to show you (using this simple image) the way I color comics using Adobe Photoshop. Tutorials about coloring are so diverse and differents, cause everyone have his own style...
0. Initial steps.
I've choose this Batman from Roger Cruz.
This penciller is one of my favorites. The very first step is open the
image, set this to RGB mode (I usually work in RGB/CMYK mode) changing
the "Background" into a layer n' set it to "multiply" mode. We can call
this one "lineart".
1. Flats
Here
we began to color... I've created a new layer under the "lineart"
called "color". This one is a normal layer mode, and we need to
establish the colors for all the image using only flat colors at this
time.
2.Alpha Channel
With
all the flat colors done, I'm going to select all the image (ctrl+A)
copy (ctrl+C) and paste it (ctrl+V) into a new Alpha layer... We need
it for select every color area in the future.
3. Airbrush
Now, using the brush tool 
with a soft brush in screen mode I paint some parts of the image,
setting the "light source" for Batman. I use the magic wand tool 
for select the different areas in the "Alpha channel" like active areas
to paint over. You need to paint-over using the same color of the part
to paint. It may looks like this:
4.Cuts
Now, the next step is where the detailed work is. Using the lasso tool
we do selections ("cuts"), and paint this areas using a gradient (using
the base color like initial color and finishing in transparent). The
gradient tool
should be in "screen" mode and 35% of opacity. We do the selection,
fills with gradient and make the selection again (more small) and fills
again. I usually try 2 or 3 times for every area.
You can change this values for every tool... It depends of you!
Experiment, and use the tool and values you feel more comfortably.
5. Secondary Light source and Adjustment
When all the Batman are fully colored, a secondary light source helps
to create more realistic effect in the image. I've choose a pale yellow
tone, and using a soft brush (in screen mode) I add some light spots
here and there.
Its almost finished... But I wanna do some adjustments for the colors:
6. Final Image
Ok, it's all for now. Hope u all find useful this tutorial too.
Remember to practice and take some time to experiment and develop your
own style. If you want to send me a word, you can use the form in the
Info section.
Hands to the coloring and God Bless you all!
