My usual coloring technique.
First, start off with a rough sketch, using a light color. I like using red for this step, because it stands out so much, but you can use any color as long as it's light.
Now add another layer- MAKE SURE YOU KNOW WHICH LAYER YOU'RE DRAWING ON AT ALL TIMES. I've had to redo a lot of lineart because I forget which layer I'm on.
For the lines, I like to put the pen mode on 'soft', and make some part thicker than others-- it makes a cool effect, and adds a little life to the lines.
Now delete the sketch layer, and use the eraser tool to fix any lineart mistakes.
Now, add a layer underneath the lineart and draw in the background. If it's a complicated background, it may need more than one layer to get it right, but if you have a nice simple/abstract bg like this one, just a single layer is needed.
PS: I really really love the blur effect for backgrounds. I didn't do it for this one, but sometimes it makes the subject more visible if the background is slightly blurry.
Now add the flat colors. It's helpful to put them all in different layers according to color ('Red', 'Blue, 'Black', 'Magenta', etc.) and do it that way, but you could just do it by article of clothing or something. Layers also help if you mess up and go too far outside the lineart.
Also, make sure that the color layers are underneath the lineart layer at all times.
Now's the fun part. :) Make another layer above everything, even the lines, and put it on 'multiply'. Then use the pen tool and a light color and color in where the shadows go.
Then, add a layer above the shadows and put it on 'add'. Pick a medium to dark shade of gray, and color in where the highlights go.
Add in the finishing touches, like the pokebeam and eyes and such.
Then you're done!
If you want to add some darker shadows, add either another multiply layer or a linear light layer and put those in.
But whatever you do, just have fun and explore. My way isn't the only way to draw-- in fact, there are several types that I use. Look at the way your favorite artists draw, and use tips and tricks from friends and fellow artists.
Thanks for reading, and I hope this helps! :D