Yeeh! I really wonder, why You want to match the existing UV pattern. It’s a really simple one. So just set your seams, unwrap, store the UV-picture as whatever U want. Open it within a paiting programm, fill colour in (you just could use the existing one as a colour-picker-source) and boom everything is like before.
Or you paint within blender with texture paint, directly on the mesh.
- mark seems along the edges, where the colour changes and unwrap and set the resolution and name of the uv-picture file.
- in edit mode, select all faces with the same colour.
- go to texture painting (in the uv-editor window, you see only the mesh part, you selected)
- choose the paint brush and the colour on the colour reel
- paint on the mesh in 3d view window (turning around) until you see in the uv-editor window the whole part solid filled)
move on with 2. to 5. until all colours are set.
Last save the uv picture.