Help please! DIY Paint Kit

[J G 927]

I am trying to build a paint kit for an aircraft that has not got one.

 

The aircraft has a mottled "skin" and I am trying to create a layered image with the various details in them.  

 

So far I have:

• Created a base layer of the overall colour.
• Extracted the easily selectable parts of the image to an otherwise transparency layer.
• Selected parts of the image that are obviously different by colour to a separate transparent layer. (Manual selection)

What I need to do next is:

• To pull out details such as lines and rivets to a separate layer
• To pull out any camo colours - these are a different shade of the main base level.

These two are hard to do as:

• The camo colours are similar to the lines and rivets colour
• The dirt has a sprayed effect and is therefore blended to the base colour.

I use the Gimp for my image building/editing, and a separate tool called DXTBmp for image conversion where required.

 

Any help on techniques would be good.