[How to edit the fin number on GIMP?]

Cool!

 

[How to edit the fin number on GIMP?]

You are talking about layer multiplication. 

That could be quite taff here.  The only way to do this is to paint that part by yourself. 

Copy the white paintkit and lay it over the norwegian paint. Then cut everything except the Gear doors so that they are white now.

Create a new layer with options "Multiplicate".

Then paint the Nose Gear Doors red like on the original. It will take time to find the right color due to the multiplication.

Then add your registration - set the layer option to "multiplicate" too. 

This would be the same progress like a normal paintjob...

[How to edit the fin number on GIMP?]

Hey! Don´t worry. GIMP is like Photoshop for  painting if you know how to use it
 

1. Import the fuselage file to your gimp.

2. Mark a part of  the red color with a box or manually. 

3. Copy that part.

4. Paste it and cover the old Registration as long as you can´t see the registration anymore.

5. Then just insert your Registration!

If you just want to change the "C" to a "G".

1. Mark the color of the "C" with the pipette.

2. Mark the "C" to a "G" 

3. Fill in the marked "Box" with the color you took from the "C".