GIMP :: Changing Train Livery Colour?

how to change a train's livery (colour) to another colour that I created with a template livery I made in an external program, leaving in things like windows and reflections in there, but in the new colour.

The problems generally happen with vehicles with multiple colours As an example of what I mean, I'd like to change the colour of this [URL] into something like [URL] I've been looking at various tutorials about how to do this, including many youtube videos, which detail how to change the colours of cars, but they mainly involve anther way including the colorize, which would do the entire vehicle based on HUE, which in this case, wouldn't work due to the multiple colours / shade the original vehicle has, including text.

Other methods use hard light to project various external images onto the body, but this mainly has a large amount of colour fluctuation with the new paint. This method generally gives the best impact when the entire image is white, doesn't say much when I am trying to preserve things like reflections and shading.