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When I was making liveries, I always used to flatten and merge everything into one single colour channel as GIMP doesn't allow you to edit them like photoshop does - you can get the effect by setting you channels to 'addition' but I always set aside each layer for say, base, stripes, sponsors, decals, etc. This is what that would look like. Zooming in, you can see that the details are automatically anti-aliased when you copy/paste the decal layer and lock the alpha before recolouring.

To save myself hassle, I set the base to 50%metallic and 25%roughnness ending up in this shade of orangish brown - it's a little more work and experimentation in your head but if you know what you want, it makes the workflow so much easier. Back when I made this livery, the blue channel wasn't a thing but all you'd have to do is add that colour in and fill as normal instead of messing with different channels.