Either way I'm going to get you to create a layer of background white (ctrl+shift+N, select all, fill with white) which we aren't going to touch. It will merely act as a placeholder so we don't have those dumb checkerboards showing through. Now take your line art and set it on top with it's "Layer Mode" set to "Multiply". From here on out your line work will remain on top and all subsequent layers get sandwiched between the white and the lines. It's so simple that you probably just shat yourself.
First we must decide how many major areas of colour there are. This is going to determine how many layers we need. Is your drawing of a bald naked person? Then you will need very few layers - one for skin and one for background. Is your drawing of a collage of intricate costume wearing Cirque De Soleil performers? Then you can expect to have a lot of layers for each distinct area of colour
In my case there are some obvious distinctions - the main uniform jacket and pants, the straps and gloves, the hair, and the skin.
I add new layers for all these areas and name them accordingly. Names are very important when you start making a lot of layers, photoshop even lets you assign different colour schemes to your layer palette to differentiate things.
Set all your layers to 'multiply'. Or don't. But just in case you forgot to put your line art on top this will let the lines show through.
The reason I hate the paint bucket tool is that won't always fill cleanly inside all linearts. If there is any area of greyish in your linework then the paint bucket usually won't fill it and instead leaves an ugly-as-rotting-fruit blotchy area. I set the tolerance as low as I can without the paint spilling over the lines and also I like to click twice when filling so that it fills in a little more completely.
Things don't need to be perfect just yet.
For ease of use I like the keyboard shortcuts "b" for the brush and "e" for the eraser, as well as " [ " and " ] " for increasing and decreasing brush size. Which is tonnes faster than using the brushes palette.
For extra speed use "shift+[" and "shift+]" for tuning up and down the hardness of your brush edge.
In you layers palette do you see the little thumbnail image of your recently brushed in layer? Well hold 'Control' and click on that thumbnail. The dancing ants have come to congratulate you on selecting your layer!
Now you need only press "Control+Shift+I" to select inverse and voila! You can now proceed to your next layer/area of colour without having to worry about overlapping anywhere you painted in your previous step.
If you get sick of the dancing ants simply press "Control+H" to hide or unhide them.
The more layers you block in the more you can add to your un-selection. Simply select the first layer with 'Control' click and then subsequent layers with 'Control+Shift' click. And then again invert your selection and move on.
Once you are done we are ready to turn the power of computer awesomeness into something so wicked you teem with respect for Charles Babbage.
Select each of your layers and hit the little checkerboard in turn to 'lock' the transparency! That's it - all your kindergarten training of colouring inside the lines was just made obsolete! You can now draw on each of your layers and you will never spill outside of the areas you have already blocked in.
Mouse users will simply have to spend more time making use of the "Flow" setting for their brushes.
Tablet users need only go to the brushes palette under "Other Dynamics" and set the opacity and flow controls to "pen pressure".
I imagine anyone reading this at least has a hobbyist's interest in digital colouring and I highly recommend investing in a tablet to help you enjoy this wonderful process even more.
If you are colouring your own work than you probably have an idea of the lighting you want already. If you are colouring someone else's work then perhaps they had an inkling as to the lighting when they started drawing and have included some shadows in the linework which you can use to guide you in your shading.
There are several ways to approach the initial shading. I start with my medium tone and then pick a lighter and a darker shade to switch between. Avoid using white and black, sure they do the job, but they do the job like a dog trying to mow the lawn - it will be bad and probably involve a lot of blood and exorbitant veterinary bills.
As a general rule I make shadows less saturated and highlights more saturated. Use your own judgment. If it looks good - do it. In this kind of work looking good is all that matters.
The advantages of this method are clear - it takes much less time and gives a much more defined shape to everything, all of which can be refined later.
You may even want to reduce the resolution of your image and do a couple digital thumbnails quickly to find a good lighting scheme. Working in black & white at that stage can also help. Even when I start in full colour I almost always change the initial shading colours by the time I am finished.
Also at this point if you feel like some of your shading is too hard and you want smoother transitions simply grab the 'smudge' tool and go at 'er with a soft touch. I just wiggle the tool around on top of areas I want to smooth. Stay away from the 'blur' tool since photoshop's idea of 'blur' at '100%' is more around the lines of, "I don't want to blur anything unless you are prepared to wait a million years for this goddamn tool to do anything close to what you would call blurring".
Now take a break to pet the nearest bunny or kitty.
The dog doesn't get pets until it finishes skimming the pool.
When you are ready Go On to Part 2