There are many ways to color an image. This option works for inked images with good contrast. We work by duplicating layers. One layer for the ink drawing and another for the color.
1. Open a black and white image.
I scanned this image in grayscale.
2. Fix any error and clean the image
You can use pencil tool or eraser to delete any imperfection in your drawing.
3. Add alpha channel
Check whether the image has an alpha channel. This will allow transparency in the image. If not, you can right click on the layer and select add Alpha Channel.
Threshold puts the image in black and white version, enhancing contrast. Filling with color will be much easier if the image has a good contrast, with no shading areas.
5. Select by color
Use select by color tool and click on an white area, then you have all white areas selected. Click delete or cut (control + X).
You end up with an image which have the trace of your drawing and transparent areas.
6. Duplicate layer
You will have two layers. The upper layer will hold the black and white trace or drawing. The second lower image will be colored.
7. Paint with bucket fill or pencil
Note: If you want to color this image you have to change its mode into RGB (a mode which supports color). Go to (Image/mode/RGB).
Coloring process is starting.
Paint by filling transparent spaces or islands.
8. (optative) Use a third layer for difficult parts.
When you want to fill several spaces or islands, you can use a third layer below color layer, and using pencil tool to color.
Here i am filling the hand of the character.