I recently learned how to digitally color art. Here are some of my recent works:
The rightmost one is the newest, and is the best. I didn't draw the sketches though. Those were done by an artist named Animeeny. I just took the sketches and colored them. I used GIMP to do my work. It's a free alternative to Photoshop and has plenty of functionality.
For the first drawing all I did was upload the penciled sketch, play around with the color levels until the lines looked dark enough and then filled in the colors. As you can see, it looks terrible.
The other three drawings took a lot more time. I had to go over the original sketch with a path tool to obtain a very clean line art. I then filled the colors in. It can be a very long process; the last drawing took about 6 hours to finish. Despite all the hard work, I actually enjoy this. When I'm working on a drawing, it almost feels like I am tying the damsel up in a way.
If I have time, I plan to color more of Animeeny's artwork.