A couple weeks ago I did a drawing tutorial. A few people mentioned that they would like to see how I do coloring also. It is actually very simple. Below is an animated gif demonstrating each step (click it to download).
Here is an explanation for each step:
1. This is how the image starts -- as a grayscale drawing. I brightened it using a brightness/contrast adjustment layer because my coloring method will make it darker again.
2. Here I applied a gradient map adjustment layer. This is a really important step to make colors look less flat when working from grayscale.
3. Above is the gradient I picked -- each shade of my grayscale image is changed to its corresponding color on this gradient. In this case, black stays black, dark grey becomes dark purple, and white becomes neon green. The colors don't have to be natural yet -- I just pick contrasting colors that will add depth to the final image. If I wanted to be more naturalistic, I would pick dark orange on the left and pale blue on the right. Anime concept artists often use dark purple on the left and light orange on the right. If these colors are too saturated, I just tone down the whole layer later.
4. In this step I pick some new unnatural, contrasting colors and paint them on an 'overlay' layer with a few fat strokes. I then soften them with with gaussian blurs or by smudging them around. This will combine with the gradient map to help add a great sense of warmth and depth later.
5. Finally on a 'multiply' layer I paint in the basic natural colors. Nothing too saturated, but otherwise what you would expect: tan for fur, brown for leather, red/brown for wood. This would look cartoonish by itself, but the contrasting colors I added earlier give them some more complexity and interest.
That's all there is to it. Some people will say you should work in color from the start. That may work great for them, but after trying for many years, I decided that I like working out the basic shapes in grayscale first. It took a lot of tries to find a technique that gave the colors a nice feel, but it's not at all hard to do once you know the basic idea. You can see it in action in all my timelapse concept art videos, like these rat, and dog concept drawing videos.
You can check out my PSD file for this image here!