How to create a transparent color from a solid color using GIMP?

As I built the web I found necessary to change the solid color from an image into the transparent color. This way I could use my image with all kind of background colors.
How can we change the solid color from an image into the transparent color using the GIMP program from Linux?

First of all we will open the file with the GIMP and we will choose the area that we want to change (for example we may choose the solid color with
TOOLS-> Selection Tools -> Choosing Colors )

Once we have a mask with the desired area, we will right click in the image and choose
LAYER -> Transparency -> “Add Alfa Channel”.
Now we can choose the Eraser Tool and drag it through our image. The masked area will show the gray rectangles that identify the transparent color.

Note: It is important to save the file with a format that allows transparency, like PNG or GIFs (the JPEG for example don't allow it).