I know that there is already a tutorial about it for photoshop users this one is for gimp users.
Let's get start
First open the manga page or panel as a new layer
Scale the images to 150x300 or any size you want ( Here I tried to make an avatar from manga panels )
Then Layer - To image size
Before going any further you should do this. Change the image mode to RGB. For that click on image and select mode. From mode select ' RGB'
Select the layer that have manga panel in it. Then click on layer from menu bar , click on transparency from transparency choose 'color to alpha'
Then a box will appear just like the image below in spoiler tag. All you have to do is click Ok
You can see that I have hide the background layer and only outer line of Naruto can see.
So far is the how to remove white color from manga panels. From here on you can choose different colur and fill it in the background layer.
I choose a grey colur and filled in thebackground layer.
You can edit image seasily. This is the most important advantage of this.I selected layer that have manga panel and used eraser.
You can see that I removed the dots in the layer.
Yes the tutorial is over. With this you can easily color the manga pages and remove unnecessary parts.